hereinafter , the present invention will be described by way of preferred embodiments with reference to the attached drawings . fig1 through fig1 are views illustrating an all terrain vehicle according to a preferred embodiment of the present invention . in this preferred embodiment , the terms “ front ”, “ forward ”, “ rear ”, “ rearward ”, “ left ”, “ right ”, “ behind ” and “ side ” refer to directions from the point of view of a rider sitting on a seat of the all terrain vehicle . in fig1 through fig1 , reference numeral 1 represents an all terrain vehicle according to the present preferred embodiment . the all terrain vehicle 1 preferably has the following structure . with reference to fig1 and fig2 , the all terrain vehicle 1 preferably includes a double - cradle type body frame 2 . by left and right ends of a front edge of the body frame 2 , left and right front wheels 3 are respectively supported via left and right front suspension devices 4 , so as to be swingable upward and downward . each front wheel 3 has a balloon tire having a low air pressure and has a large width . by left and right ends of a rear edge of the body frame 2 , left and right rear wheels 5 similar to the front wheels 3 are respectively supported via a rear suspension device 6 , so as to be swingable upward and downward . in the cradle of the body frame 2 , an engine unit 7 is mounted . above the engine unit 7 , a fuel tank 8 is mounted . forward of the fuel tank 8 , a steering shaft 9 for steering the left and right front wheels 3 is provided so as to be pivotable rightward and leftward . a steering handle 10 is fixed to a top end of the steering shaft 9 . rearward of the fuel tank 8 , a seat 11 is provided . a front portion of the steering shaft 9 is covered with a front cover 12 . left and right portions of the fuel tank 8 and lower left and right portions of the seat 11 are covered with side covers 13 . above the left and right front wheels 3 , left and right front fenders 14 are provided . above the left and right rear wheels 5 , left and right rear fenders 15 are provided . as shown in fig3 , each front suspension device 4 includes top and bottom front arms 4 a and a cushion unit 4 b . the top and bottom front arms 4 a are supported by a front portion of the body frame 2 so as to be swingable upward and downward , and support the corresponding front - wheel 3 . the cushion unit 4 b is located between the bottom front arm 4 a and the body frame 2 . as shown in fig4 , the rear suspension device 6 includes a rear arm 6 a and a cushion unit 6 b . the rear arm 6 a is supported by a rear portion of the body frame 2 so as to be swingable upward and downward , and supports the rear wheels 5 . the cushion unit 6 b is provided between the rear arm 6 a and the body frame 2 , and is located at the center in a width direction of the all terrain vehicle 1 . in this specification , the “ width direction ” is a direction that is substantially parallel to the phantom straight line connecting the centers of the left and right front wheels 3 or the centers of the left and right rear wheels 5 . the “ longitudinal direction ” is a horizontal direction that is substantially perpendicular to the width direction . the seat 11 has the structure shown in fig1 . a cushion material 11 b is provided on a bottom plate 11 a . the cushion material 11 b is covered with a surface layer 11 c , and a periphery of the surface layer 11 c is fixed to a periphery of the bottom plate 11 a . the bottom plate 11 a has a groove 11 d extending along each of left and right edges thereof . each groove 11 a extends in a longitudinal direction of the all terrain vehicle 1 , and is recessed upward . the left and right rear fenders 15 each have a gutter - like groove 15 a extending along the corresponding side edge of the seat 11 . each gutter - like groove 15 a is provided for allowing rainwater or the like to flow in the longitudinal direction . an inner wall 15 b of each rear fender 15 protrudes upward like a rib . each inner wall 15 b is located in the corresponding groove lid of the bottom plate 11 a of the seat 11 . as a result of the features of the above - described structure , rainwater or the like dropping on the rear fenders 15 flows in the grooves 15 a in the longitudinal direction and thus , substantially never flows to an area below the seat 11 . the body frame 2 includes a main frame 16 ( fig1 ) for supporting the engine unit 7 , the front suspension devices 4 and the rear suspension device 6 , and a seat rail 17 ( fig5 ) attached to the main frame 16 for supporting the seat 11 . the seat rail 17 is provided for bearing the load acting on the seat 11 , for example , the body weight of the rider . the seat rail 17 is detachably provided on a rear arm supporting section 16 a of the main frame 16 . the seat rail 17 is preferably molded of an aluminum alloy , and preferably has a substantially rectangular shape when viewed from above as shown in fig7 . the seat rail 17 includes a left rail main body 31 a and a right rail main body 31 b both extending in the longitudinal direction , a rear portion 31 c for connecting rear ends of the left and right rail main bodies 31 a and 31 b , and a front portion 31 d for connecting front ends of the left and right rail main bodies 31 a and 31 b . the front portion 31 d is curved to protrude upward like a gate . on a top surface of the front portion 31 d , supporting bosses 31 e are formed for supporting a rear portion of the fuel tank 8 . to a rear portion of the seat rail 17 , a rear bumper 74 is attached . the rear bumper 74 is preferably formed by bending a metal pipe into a substantially c - shaped configuration having left and right portions 74 a and a rear portion 74 b , and then forming attachment bosses 74 c respectively at front ends of the left and right portions 74 a . the rear bumper 74 is arranged to partially surround the rear portion of the seat rail 17 from behind when viewed from above , and to slightly incline upward and rearward with respect to the left and right rail main bodies 31 a and 31 b when viewed from the side of the all terrain vehicle 1 . the attachment bosses 74 c of the rear bumper 74 are detachably attached to rear portions of the rail main bodies 31 a and 31 b by bolts 74 d . as best shown in fig5 , the engine unit 7 includes a crank case for accommodating a crank shaft , and a cylinder block 7 b , a cylinder head 7 c and a head cover 7 d which are stacked on a top wall of a front portion of the crank case . the crank case , the cylinder block 7 b , the cylinder head 7 c , and the head cover 7 d are tightened to each other by bolts . a front wall of the cylinder head 7 c has an exhaust port , and an exhaust pipe 32 a ( fig1 ) of an exhaust system 32 is connected to the exhaust port . the exhaust pipe 32 a extends forward , is bent right - rearward , passes on the right of the cylinder block 7 b , and extends rearward below the right rail main body 31 b of the seat rail 17 . as best shown in fig1 , a muffler 32 b having a larger diameter than that of the exhaust pipe 32 a is connected to a rear end of the exhaust pipe 32 a . the muffler 32 b passes below the right rear fender 15 and extends along the right rail main body 31 b . the muffler 32 b is fixed to the right rail main body 31 b by a bolt 32 c via a bracket 32 d . the muffler 32 b preferably has an elliptical cross section which is longer in the horizontal direction than in the vertical direction ( fig1 ). as a result of such a shape , the muffler 32 b has a necessary volume as a muffler and still is prevented from protruding downward . thus , the upward and downward swinging strokes of the rear wheels 5 can be guaranteed by the cushion device 6 b while the height of the body of the all terrain vehicle 1 in the upward - downward direction is limited . as best shown in fig5 , a rear wall of the cylinder head 7 c has an intake port , and a throttle body 33 a of an intake system 33 is attached to the intake port via a joint member 33 e . in the throttle body 33 a , a throttle valve is provided for controlling the dimensional area of an intake passage . a fuel injection valve 33 b is provided on a top wall of the throttle body 33 a so as to incline up rearward . the fuel injection valve 33 b injects the fuel toward the intake port . a fuel supply rail 33 c extending generally horizontally in the width direction of the all terrain vehicle 1 is connected to a fuel introduction opening formed at a top end of the fuel injection valve 33 b . one end of the fuel supply rail 33 c is connected to a discharge opening of a fuel supply pump 34 provided in the fuel tank 8 via a fuel supply hose 33 d . a portion of a bottom wall of the fuel tank 8 which corresponds to a bottom end of the fuel supply pump 34 protrudes downward . the protruding portion is located in a space formed by the head cover 7 d of the engine unit 7 , the throttle body 33 a , and fuel injection valve 33 b and the like . as best shown in fig6 , an air cleaner 71 is connected to the throttle body 33 a via an intake duct 35 . the intake duct 35 is preferably formed of a resin or rubber , and is gradually bent downward from the throttle body 33 a and then is bent rearward . the intake duct 35 is located between the left and right rail main bodies 31 a and 31 b and on the right of the center line of the all terrain vehicle 1 in the width direction , when viewed from above ( fig6 ). on the left of the intake duct 35 , a battery 83 is provided . the battery 83 is accommodated in a supporting box 84 . the supporting box 84 is arranged so as to bridge between the left and right rail main bodies 31 a and 31 b . the battery 83 is fixed to the supporting box 84 via a bracket 85 , and an insulating cover sheet is provided between the battery 83 and the bracket 85 . reference numeral 86 represents a starter relay . reference numeral 87 represents a cover sheet for covering the battery 83 . the air cleaner 71 is preferably a generally parallelepiped box formed of a resin . the air cleaner 71 includes an air cleaner case 72 and a filtering element ( not shown ) provided in the air cleaner case 72 . as best shown in fig1 , the air cleaner case 72 includes a case main body 72 a defining a lower portion and a case lid 72 b defining an upper portion . on left and right walls of the case main body 72 a , flanges 72 c extending upward and then outward are respectively provided . the flanges 72 c are preferably integrally formed with the case main body 72 a . the flanges 72 c are located on the left and right rail main bodies 31 a and 31 b , respectively and are fixed to the left and right rail main bodies 31 a and 31 b by bolts 72 d . an air introduction duct 72 e ( fig5 ) is integrally formed with the case lid 72 b . as shown in fig5 , the air introduction duct 72 e extends upward and forward obliquely from the case lid 72 d . in more detail , the air introduction duct 72 e extends forward obliquely along the front portion 11 a ′, which inclines upward and forward obliquely , of the bottom plate 11 a of the seat 11 . an opening 72 f of the air introduction duct 72 e is located at about the same height as a top surface of the engine unit 7 . as best shown in fig1 , an undercover 73 is provided below the air cleaner 71 . the undercover 73 is preferably formed of a resin , and includes a case cover portion 73 a for covering right , left and bottom surfaces of a rear portion of the air cleaner 71 and a rear extension portion 73 b extending rearward from the case cover portion 73 a for covering right , left , bottom and rear surfaces of the body of the all terrain vehicle 1 , which is rearward of the air cleaner 71 . as shown in fig1 , the cross section of the undercover 73 taken along a plane crossing the axis ( longitudinal direction ) of the all terrain vehicle 1 preferably has a substantially c - shaped configuration which is open at an upper portion thereof , when viewed from behind . the undercover 73 includes left and right walls 73 c and 73 d , a bottom wall 73 e , and a rear wall 73 f ( fig1 ). as best shown in fig1 , top edges 73 c ′ and 73 d ′ of the left and right walls 73 c and 73 d of the undercover 73 are mostly inside the left and right rail main bodies 31 a and 31 b of the seat rail 17 . when viewed from the side of the all terrain vehicle 1 , the top edges 73 c ′ and 73 d ′ overlap the left and right rail main bodies 31 a and 31 b . when viewed in the cross section in fig1 , the left and right rail main bodies 31 a and 31 b each have a horizontal portion a and a vertical portion b extending downward from an outer edge of the horizontal portion a , the outer edge being outer in the width direction of the all terrain vehicle 1 . the top edges 73 c ′ and 73 d ′ are each located below the corresponding horizontal portion a and inside the corresponding vertical portion b . the case cover portion 73 a is provided mainly for preventing rainwater or the like splashed upward from colliding with , and going up along , a rear wall 71 a ( fig1 ) or left or right wall of the air cleaner 71 , and thus flowing toward the opening 72 f ( fig5 ) of the air introduction duct 72 e . for this purpose , the left and right walls 73 c and 73 d and the bottom wall 73 e of the case cover portion 73 a extend forward from the rear wall 71 a of the air cleaner 71 and cover the rear portion of the air cleaner 71 as shown in fig1 . the left wall 73 c and a left portion of the bottom wall 73 e of the case cover portion 73 a extend further forward as compared to the right wall 73 d . this extended portion will be referred to as an “ extension portion 73 g ”. above the extension portion 73 g , a control unit 75 ( fig6 ) for controlling the engine driving state and the like of the all terrain vehicle 1 in this preferred embodiment is provided . since the extension portion 73 g prevents rainwater or the like from colliding with the air cleaner 71 , the rainwater or the like can be prevented , with certainty , from going up along the left portion of the air cleaner 71 and dropping on the control unit 75 . inside a rear portion of the rear extension portion 73 b of the undercover 73 , a tail light 76 is accommodated . as shown in fig8 , the tail light 76 preferably has a generally parallelepiped shape that is longer in the width direction of the all terrain vehicle 1 . as shown in fig8 and fig9 , the tail light 76 includes a lens 76 a and a supporting plate 76 b for supporting the lens 76 a . on the supporting plate 76 b , left and right supporting bosses 76 c are formed so as to protrude in the opposite direction from the lens 76 a . each supporting boss 76 c is tightened and fixed to a lamp bracket 73 h , provided to stand on the undercover 73 , by a bolt 77 . the lens 76 a is directed rearward through an opening 73 i formed in the rear wall 73 f of the undercover 73 . accordingly , when viewed from behind the all terrain vehicle 1 , the lens 76 a appears to be mounted on the rear wall 73 f of the undercover 73 . this structure can give an impression that the body of the all terrain vehicle 1 and the tail light 76 are integral and thus improves the external appearance . reference numeral 78 represents a cable for supplying electric power to the tail light 76 . one of the lamp brackets 73 h is engaged with a clip 79 , and the cable 78 is supported by the clip 79 . reference numeral 80 represents a working hole formed in the bottom wall 73 e ( fig1 ) of the undercover 73 . around the working hole 80 , a substantially cylindrical wall extending upward from the bottom wall 73 e is provided . as a result of such a structure , the bottom wall 73 e is prevented from being reduced in rigidity by the formation of the working hole 80 , and rainwater or the like is prevented from entering the working hole 80 . the working hole 80 is effectively used when the tail light 76 is tightened and fixed to the lamp brackets 73 h by the bolts 77 . the undercover 73 is attached to the body of the all terrain vehicle 1 as follows . as shown in fig1 , each of left and right stopping bosses 73 j ( only one stopping boss 73 j is shown in fig1 ) is provided in a rear portion of the undercover 73 , and an engaging slit 73 k of the stopping boss 73 j is engaged with a grommet 74 f provided on an engaging pipe 74 e located behind the engaging slit 73 k . front edges of the left and right walls 73 c and 73 d of the undercover 73 are tightened and fixed to side walls of the case main body 72 a of the air cleaner 71 by a bolt 81 ( fig1 ). as shown in fig6 , a tool 82 a , an air gauge 82 b , and electric components such as a relay 82 c , a fuse 82 d and the like are provided between the rear portion 31 c of the seat rail 17 and the rear wall 71 a of the air cleaner 71 . these vehicle - mounted components are accommodated in an accommodating box , and left , right and bottom surfaces of the accommodating box are surrounded by the undercover 73 . since the accommodating box is not seen from outside , the degree of freedom of arranging the vehicle - mounted components is increased and the undercover 73 of a simple shape improves the external appearance . the vehicle - mounted components are not limited to the components described in this preferred embodiment , and may be electric components including a battery and a control unit . according to this preferred embodiment , the air cleaner 71 is located below the seat rail 17 , which is located below the seat 11 , so as to be positioned between the left and right rail main bodies 31 a and 31 b when viewed from above , and the left , right and bottom surfaces of the air cleaner 71 are covered with the undercover 73 . therefore , rainwater dropping from above and also rainwater or muddy water splashed from the rear wheels 5 can be prevented from entering an inner portion of the air cleaner 71 . since the muddy water or the like is prevented from going up along the side surfaces of the air cleaner 71 by covering the bottom surface of the air cleaner 71 with the undercover 73 , the problem of the complicated structure which is caused by the labyrinth structure does not arise . in addition , since the left and right surfaces of the air cleaner 71 are covered with the undercover 73 , the external appearance of the all terrain vehicle 1 when viewed from the side is improved . the air cleaner 71 is supported by the left and right rail main bodies 31 a and 31 b , and the top edges 73 c ′ and 73 d ′ of the left and right walls 73 c and 73 d of the undercover 73 are located inside the left and right rail main bodies 31 a and 31 b so as to appear to overlap the left and right rail main bodies 31 a and 31 b when viewed from the side . therefore , rainwater or muddy water can easily flow down along the left and right rail main bodies 31 a and 31 b and the undercover 73 , and thus can be prevented more certainly from entering the inner portion of the air cleaner 71 . in the rear portion of the undercover 73 , the rear extension portion 73 b covering the right , left , bottom and rear surfaces of the rear portion of the body of the all terrain vehicle 1 is provided . inside the rear extension portion 73 b , the tail light 76 is accommodated . the lens 76 a of the tail lamp 76 is directed rearward through the opening 73 i formed in the rear wall 73 f of the rear extension portion 73 b . as a result of such a structure , rainwater or the like can be prevented from being splashed against electric lines of the tail light 76 , and the area around the tail light 76 appears simple and neat so as to improve the external appearance . to the rear portion of the seat rail 17 , the rear bumper 74 is attached and arranged so as to surround the rear portion , and the rear extension portion 73 b of the undercover 73 is attached to the rear bumper 74 . therefore , the rear portion of the undercover 73 can be supported by the rear bumper 74 , which is originally provided for protecting the rear end portion of the body of the all terrain vehicle 1 . thus , an increase in the number of components can be avoided . the tail light 76 is attached to the lamp brackets 73 h which are preferably integrally formed with the rear extension portion 73 b of the undercover 73 . such an arrangement eliminates the necessity of separately providing a supporting member for the tail light 76 , and thus simplifies thus the attaching structure of the tail light 76 . the above - described arrangement also makes it easy to positionally align the lens 76 a of the tail light 76 and the opening 73 i of the undercover 73 . the reason is that the lamp brackets 73 h are formed on the undercover 73 which has the opening 73 i , and the tail light 76 is attached to the lamp brackets 73 h . the tool 82 a and the electric components including the relay 83 c are provided rearward with respect to the air cleaner 71 and forward with respect to the tail light 76 , and are covered with the rear extension portion 73 b of the undercover 73 . therefore , the tool and other electric components can be protected against rainwater and muddy water . the left , right and bottom surfaces of the portion of the all terrain vehicle 1 from the air cleaner 71 to the tail light 76 are covered with one portion of the undercover 73 . the top edges 73 c ′ and 73 d ′ of the left and right walls 73 c and 73 d of the undercover 73 are located inside the rail main bodies 31 a and 31 b in the width direction of the all terrain vehicle 1 , and appear to overlap the rail main bodies 31 a and 31 b when viewed from the side of the all terrain vehicle 1 . therefore , rainwater or the like splashed from the rear wheels 5 is prevented from entering inside the undercover 73 , and also the external appearance is improved . the present invention is not limited to the above - described preferred embodiments , but various variations and modifications may be made without departing from the scope of the present invention . the presently disclosed preferred embodiments are therefore considered in all respects to be illustrative and not restrictive . the scope of the present invention is indicated by the appended claims rather than the foregoing description , and all changes which come within the meaning and range of equivalence thereof are intended to be embraced therein .