in the following , an embodiment of the present invention is described on the basis of the drawings . fig1 to 5 exemplify a reduced tail - swing backhoe as a working machine 1 , and in the backhoe 1 is adapted to : on crawler - type traveling units 2 , support a machine base ( revolving base ) 3 that is revolvable around a vertical shaft ; have a bucket type excavator unit 4 on a front side of the machine base 3 ; and on the machine base 3 , mount a cabin 7 surrounding a controlling part 6 having an operator &# 39 ; s seat 5 . the machine base 3 is adapted to : mount an engine 11 on a back part of a revolving frame 3 a ; mount vehicle accessories 12 such as an oil tank , a control valve , and a battery on one of left and right sides ( on a right side ); cover the engine 11 with a fixed engine cover 13 and an openable / closable engine hood 14 ; and cover the vehicle accessories 12 with an accessory cover 15 . the accessory cover 15 has : an accessory side cover 15 a that is fixed to a laterally middle part of the revolving frame 3 a ; and an outer open / close cover 15 b that is openable / closable . the cabin 7 is substantially rectangular shaped in a plan view , mounted on a front part of the revolving frame 3 a and on a left side of the accessory cover 15 , and provided with : left and right pairs of front supporting posts 7 a and side supporting posts 7 b ; an upper frame 7 c that is connected to upper ends of the four supporting posts , i . e ., the front and side supporting posts 7 a and 7 b , and attached with a ceiling member ; a lower frame 7 d that ranges from lower ends of the front supporting posts 7 a to lower ends of the side supporting posts 7 b and is connected with the lower ends of the left and right side supporting posts 7 b ; an open / close door 7 e that opens / closes a doorway surrounded by a front supporting post 7 a , side supporting post 7 b , and frame members such as the upper frame 7 c and the lower frame 7 d ; and the like . in fig1 to 3 and 5 to 9 , in the controlling part 6 inside the cabin 7 , a seat base 21 is fixed to a longitudinal middle part of the revolving frame 3 a in front of the engine cover 13 ; on the seat base 21 , the operator &# 39 ; s seat 5 is placed ; and inside the seat base 21 , an air conditioner 20 is contained . on upper right and left sides of the seat base 21 , lever controlling devices 22 r and 22 l for controlling a boom cylinder and bucket cylinder of the excavator unit 4 and a revolving motor for the machine base 3 , and the like are arranged with being distributed right and left across the operator &# 39 ; s seat 5 . on an upper surface of the revolving frame 3 a in front of the seat base 21 , a floor step 23 is detachably provided ; in front of the floor step 23 , a front floor plate 19 is provided ; in such a way as to vertically penetrate through the front floor plate 19 , a traveling lever controlling device 24 for controlling hydraulic motors of the traveling units 2 , and the like is arranged in the front part of the revolving frame 3 a ; and also , a traveling accelerator pedal , a swing pedal for operating a swing cylinder of the excavator unit 4 , and the like are arranged . upward from a right front of the seat base 21 , an air duct 25 is protruded ; the air duct 25 is communicatively connected to the air conditioner 20 ; a plurality of air blowing ports 25 a for blowing cooling air to a windshield , operator &# 39 ; s seat 5 , and the like in the cabin 7 is formed ; and on a back side of an upper part of the air duct 25 , a meter panel 26 is provided . the air duct 25 and the meter panel 26 constitute an operation device 27 . the operation device 27 is positioned in front of the right lever controlling device 22 r , i . e ., arranged in a posture of rising from below on the right front side of the operator &# 39 ; s seat 5 , and the meter panel 26 is provided with various types of meter display parts , a maintenance display part , and the like . the accessory side cover 15 a that covers an inner lateral side of the vehicle accessories 12 forms a right side wall of the cabin 7 , and between the accessory side cover 15 a and the air duct 25 as the operation device 27 , an engine control unit ( ecu ) 30 as an electronic device that controls the engine 11 is arranged . in a right lateral part of the revolving frame 3 a , a support member 29 is provided upright . an upper part of the support member 29 supports the air duct 25 and is attached with the engine control unit 30 . the engine control unit 30 is one that controls the engine 11 by itself , or one that is , with a main engine control unit performing main control of the engine 11 being provided inside the engine cover 13 , connected to the main engine control unit to secondarily control the engine 11 . on a right lateral side of the floor step 23 , a harness guide member 32 that guides harnesses 31 from the engine control unit 30 to the engine 11 side is provided under the support member 29 , and on a left lateral side of the floor step 23 , a doorway step ( a getting - on / off step ) 34 is provided on a left machine base side cover 33 of the machine base 3 . a front part of the left machine base side cover 33 is connected to a front cover 36 that covers a left front part of the machine base 3 . on the basis of fig1 to 3 and 6 to 12 , attachment structure of the engine control unit 30 as an electronic device is described . the engine control unit 30 is supported by the support member 29 and arranged between the accessory side cover 15 a of the accessory cover 15 covering the vehicle accessories ( auxiliary devices for vehicle ) 12 on a right lateral side of the controlling part 6 and the operation device 27 having the air duct 25 and the meter panel 26 . the support member 29 is , in an upper part of a supporting post 29 a made of a round pipe , provided with : an angle - shaped inner bracket 29 b for attaching the operation device 27 ; and a vertically long outer bracket 29 c for attaching the engine control unit 30 . the engine control unit 30 is adapted such that , inside a box - shaped case cover 30 a , a unit main body 30 b as an electronic device main body is attached , and the case cover 30 a is held with being suspended at an upper part of the outer bracket 29 c through bolts , and also fixed on a lateral surface with bolts . attachment surfaces of the inner bracket 29 b and outer bracket 29 c of the support member 29 are laterally and vertically separated , and the operation device 27 is attached on the operator &# 39 ; s seat 5 side , and can be attached / detached to / from the inner bracket 29 b even in the presence of the engine control unit 30 , whereas the engine control unit 30 is attached on the accessory side cover 15 a side , and can be attached / detached to / from the outer bracket 29 c even in the presence of the operation device 27 . near a lower part of the support member 29 on the machine base 3 , the harness guide member 32 is arranged , and the harness guide member 32 is provided with a stay 41 attached to a lower part of the outer bracket 29 c . as illustrated in fig6 to 10 , 17 , and 18 , the harness guide member 32 has : a fitting part 32 c that is l - shaped in a plan view , and fits into the supporting post 29 a in a middle part ; a first insertion port 32 aa that is positioned on a left lateral side of the fitting part 32 c ; and a second insertion port 32 ab that is positioned on a front lateral side of the fitting part 32 c , and is inserted with the harnesses 31 connected to the engine control unit 30 and engine 11 or the main engine control unit to guide the harnesses 31 into the machine base 3 . surfaces forming the first and second insertion ports 32 aa and 32 ab of the harness guide member 32 are on front sides of chevron surfaces that protrude upward , and aback side tilted surface of the first insertion port 32 aa serves as a footrest surface 32 b , whereas a back side tilted surface of the second insertion port 32 ab is provided with the stay 41 . accordingly , the harness guide member 32 is attached to the lower part of the outer bracket 29 c to guide the plurality of harnesses 31 with longitudinally and laterally separating the harnesses 31 , and by providing the footrest surface 32 b , prevents a foot of an operator from being caught by any of the harnesses 31 . on the basis of fig1 to 3 , 6 to 9 , and 13 to 15 , the floor step 23 is described in detail . the floor step 23 is one formed by press punching and drawing processes of a laterally long flat plate , on which a plurality of ( two ) swelling parts 38 that are laterally long and of an upward protruding shape are longitudinally formed ; on one of left and right sides ( right side ) of the floor step 23 and on a front side of an end part of the back swelling part 38 b , an opening part 40 for inserting the harnesses 31 for the engine control unit ( electronic device ) 30 is provided ; and on a laterally middle part side of the opening part 40 , a connecting swelling part 38 c that connects the back swelling part 388 and the front swelling part 38 a to each other is formed . above the opening part 40 , the harness guide member 32 is arranged . the opening part 40 may be opened to a front end of the floor step 23 to form a swelling shape connecting a right end of the front swelling part 38 a and a front end of the connecting swelling part 38 c to each other ; however , in the present embodiment , an end part 38 aa of the front swelling part 38 a is provided with extending to a lateral side of the connecting swelling part 38 c and a front side of the opening part 40 . the front swelling part 38 a is adapted such that an end part thereof on a side connecting to the connecting swelling part 38 c is formed to have a longitudinally wide width as compared with the rest thereof , and thereby the right end part 38 aa of the front swelling part 38 a can go into the front side of the opening part 40 . the floor step 23 improves strength thereof by longitudinally forming the plurality of swelling parts 38 and also connecting the back swelling part 388 and the front swelling part 38 a through the connecting swelling part 38 c , and prevents a reduction in strength of an end part of the floor step 23 , which is caused by forming the opening part 40 , by making the right end part 38 aa of the front swelling part 38 a go into the front side of the opening part 40 . on the floor step 23 , a floor mat 39 is laid . the floor mat 39 is , on a lower surface thereof , formed with a rib that fits not only in a circumference of the floor step 23 but also fits between the front and back swelling parts 38 a and 38 b , and laid over the whole of an upper surface of the floor step 23 . in fig1 to 3 , 5 , 7 , and 18 to 21 , structure of the doorway step 34 is described in detail . the doorway step 34 is arranged in a position where the lower frame 7 d of the cabin 7 and a lower part of the open / close door 7 e overlaps with each other ; on an upper surface of the left side machine base side cover 33 , a concave portion 35 of which a bottom surface 35 a is horizontal is formed ; in the concave portion 35 , a doorway step member 34 a is provided ; and an outer edge 34 b of the doorway step member 34 a is of an arc shape that is parallel to a side surface of the left machine base side cover 33 , whereas an inner edge 34 a is positioned on an inner side of an outer surface of the lower part of the open / close door 7 e of the cabin 7 and of a substantially parallel linear shape . that is , regarding the doorway step 34 , on a front lateral surface of the revolving frame 3 a , the left machine base side cover 33 is provided ; the left machine base side cover 33 has a horizontal upper wall 33 a and an arc - shaped side wall 33 b ; and on the side wall 33 b side of the upper wall 33 a , the doorway step concave portion 35 is formed . regarding the concave portion 35 , the bottom surface 35 a is formed horizontally and in parallel with an upper surface of the upper wall 33 a ; a rising tilted surface 35 b that rises obliquely upward from a circumference of the bottom surface 35 a is formed ; and an upper edge of the rising tilted surface 35 b is connected to the upper surface of the upper wall 33 a . part of the upper edge of the rising tilted surface 35 b on the floor step 23 side ( part of the upper edge of the tilted surface on a longitudinally long depth side ) is linear , and formed on an inner side of the outer surface of the lower part of the open / close door 7 e and in substantially parallel with the outer surface of the lower part . the doorway step member 34 a is formed of resin , rubber , or the like , and with being inserted into the concave portion 35 , attached to the left machine base side cover 33 with fastening members such as bolts . the doorway step member 34 a is of a shape similar to a planar shape of the concave portion 35 , and the inner edge 34 a on the cabin 7 side is of a linear shape substantially parallel to the upper edge of the rising , tilted surface 35 b , and arranged on the inner side than the outer surface of the lower part of the open / close door 7 e and in substantially parallel with the outer surface of the lower part . the outer edge 34 b of the doorway step member 34 a is formed in the arc shape that is an outward protruding shape parallel to an outer surface of the machine base side cover 33 and an outer edge of the concave portion 35 . the doorway step member 34 a is adapted such that , in a lateral width direction thereof , an upper surface from the inner edge 34 a to a middle part is a flat surface 34 aa , and an upper surface from the middle part to the outer edge 34 b is a nonslip surface 34 ab having unevenness ; the outer surface of the lower part of the open / close door 7 e at the time of closing is positioned in the middle part of the doorway step member 34 a ; the flat surface 34 aa from the inner edge 34 a to middle part of the doorway step member 34 a overlaps with the open / close door 7 e ; and the nonslip surface 34 ab is positioned on an outer side of the outer surface of the lower part of the open / close door 7 e . note that in the present invention , it is best to configure shapes of the respective members in the embodiment , and longitudinally , laterally , and vertically positional relationships among the respective members as illustrated in fig1 to 21 . however , without limitation to the embodiment , the members and configurations may be variously modified , and combinations may be changed . for example , the air duct 25 attached to the support member 29 may be formed with an air blowing port for blowing cooling air toward the engine control unit 30 . it is to be understood that although the present invention has been described with regard to preferred embodiments thereof , various other embodiments and variants may occur to those skilled in the art , which are within the scope and spirit of the invention , and such other embodiments and variants are intended to be covered by the following claims . the texts of japanese application nos . 2012 - 174125 , 2012 - 174123 , and 2012 - 174124 filed on aug . 6 , 2012 are hereby incorporated by reference .