reference will now be made in detail to the present embodiments of the present invention , examples of which are illustrated in the accompanying drawings , wherein like reference numerals refer to the like elements throughout . the embodiments are described below in order to explain the present invention by referring to the figures . fig4 is a plan view showing the structure of a laser scanning device according to one embodiment of the present invention , and fig5 is a cross - sectional view taken along line v - v ′ in fig4 . as shown in fig4 , the laser scanning device 100 includes a light source 110 , a collimating lens 111 , slits 112 , a cylindrical lens 113 , a polygonal mirror 115 , and a focusing lens 117 . the light source 110 generates a laser beam l , and a laser diode is usually used therefore , although it is understand that other light emitting devices could be employed . the collimating lens 111 changes the laser beam l , emitted from the light source 110 , into a parallel light beam or a converging light beam . the slits 112 restrict the procession of the laser beam l . the cylindrical lens 113 focuses the laser beam l , having passed through the slits 112 , onto the surface of the polygonal mirror 115 linearly in the horizontal direction . the polygonal mirror 115 scans the laser beam l , passed through the slits 112 , by moving the laser beam l at a constant linear velocity in the horizontal direction . to this end , a motor 116 rotates the polygon mirror 115 at a constant velocity . the focusing lens 117 , as an f - θ lens , has a predetermined index of refraction with respect to an optical axis thereof . the focusing lens 117 polarizes the reflected laser beam of a constant speed from the polygon mirror 115 into the main scanning direction ( arrow a ), corrects aberration , and focuses the laser beam on the laser scanning surface of the photosensitive drum 119 . in addition , an optical sensor 121 provides horizontal sync with the laser beam l . the optical sensor 121 includes a sync signal detection reflector 123 to reflect the light , having passed through the focusing lens 117 , towards the optical sensor 121 . the above - described components are typically assembled in a single optical housing 125 of the laser scanning device 100 . the optical housing 125 is also provided with a mirror assembly unit where reflective mirrors ( a first reflective mirror 126 a and a second reflective mirror 126 b : see fig6 and 7 ) to change the path of the light , having passed through the focusing lens 115 , are assembled . referring to fig5 , the mirror assembly unit 130 includes first and second mirror supporting members 131 and 133 to support installation angles of the first and second reflective mirrors 126 a and 126 b , respectively . according to an embodiment of the invention , the first and second mirror supporting members 131 and 133 each comprise pairs of supporters 131 a , 131 b , and 133 a , 133 b , respectively , to support both upper and lower sides of the reflective mirrors 126 a and 126 b , respectively . according other embodiments of the invention ( not shown ), the first and second mirror supporting member are single structures or comprise more than two structures . the first and second pairs of supporters 131 a and 131 b and 133 a and 133 b each may comprise insertion grooves 131 a ′, 131 b ′, and 133 a ′, 131 a ′, respectively , into which the upper and lower sides of the first and second reflective mirrors 126 a and 126 b are inserted , respectively . the optical housing 125 is provided with a plurality of light outlets 125 a , 125 b and 125 c . meanwhile , glass covering supporting units 125 d , 125 e , and 125 f are formed in the vicinity of the light exits 125 a , 125 b , and 125 c . the first , second , and third glass coverings 151 , 153 , and 155 are installed through the glass covering supporting units 125 d , 125 e , and 125 f , respectively , and prevent the influx of foreign substances from an exterior of the optical housing 125 . fig5 illustrates a case in which the photosensitive drum 119 is installed in line with the focusing lens 117 . thus , light l 1 , having passed through the focusing lens 117 , is transmitted through the first glass covering 151 without changing a path thereof and is then focused on the photosensitive drum 119 . in this case , a separate reflective mirror is not required . fig6 is a diagram showing an embodiment of the invention in which laser scanning occurs above the optical housing . as shown in fig6 , a photosensitive drum 119 ′ is installed above an upper portion of the optical housing 125 . a first reflective mirror 126 a is installed at the first pair of supporters 131 a and 131 b to reflect light l 2 , having passed through the focusing lens 117 , in the upward direction relative to the optical housing 125 . the reflected light l 2 passes through the second glass covering 153 and , collides in focus with the photosensitive drum 119 ′. fig7 is a diagram showing an embodiment of the invention in which laser scanning occurs below the optical housing . as shown in fig7 , a photosensitive drum 119 ″ is installed below a lower portion of the optical housing 125 . a second reflective mirror 126 b is installed at the second pair of supporters 133 a and 133 b to reflect light l 3 , having passed through the focusing lens 117 , in the downward direction relative to the optical housing 125 . the reflected light l 3 from the second reflective mirror 126 b passes through the third glass covering 153 and , then , collides in focus with the photosensitive drum 119 ″. according to another aspect of the invention shown in fig8 through 10 , a reflective mirror 126 d is hingedly attached to the optical housing 125 using hinge 135 to allow a single mirror 126 d to provide multiple reflection directions . the optical housing 125 is provided with a plurality of light outlets 125 a , 125 b and 125 c . glass covering supporting units 125 d , 125 e , and 125 f are formed in the vicinity of the light exits 125 a , 125 b , and 125 c . the first , second , and third glass coverings 151 , 153 , and 155 are installed through the glass covering supporting units 125 d , 125 e , and 125 f , respectively , and prevent the influx of foreign substances from an exterior of the optical housing 125 . however , it is understood that other constructions are possible to provide a single reflecting mirror and a single mirror supporting member that is able to shift positions and attitudes at which the light is not reflected , reflected upward , or reflected downward . additionally , while not required , it is understood that mirror supporting members can further be included to secure the mirror 126 d in its angled positions . as shown in fig8 , the photosensitive drum 119 is installed in line with the focusing lens 117 . as such , the mirror 126 d is rotated about the hinge 135 to not be in the light path between the light source 115 and the photosensitive drum 119 . thus , light l 1 , having passed through the focusing lens 117 , is transmitted through the first glass covering 151 without the mirror 126 d changing a path thereof , and is then focused on the photosensitive drum 119 . as shown in fig9 , a photosensitive drum 119 ′ is installed above an upper portion of the optical housing 125 . the reflective mirror 126 d is rotated about the hinge to reflect the light l 2 , having passed through the focusing lens 117 , in the upward direction relative to the optical housing 125 . the reflected light l 2 passes through the second glass covering 153 , and collides in focus with the photosensitive drum 119 ′. as shown in fig1 , a photosensitive drum 119 ″ is installed below a lower portion of the optical housing 125 . the reflective mirror 126 d is rotated about the hinge to reflect the light l 3 , having passed through the focusing lens 117 , in the downward direction relative to the optical housing 125 . the reflected light l 3 from the second reflective mirror 126 b passes through the third glass covering 153 and collides in focus with the photosensitive drum 119 ″. although a few embodiments of the present invention have been shown and described , it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention , the scope of which is defined in the claims and their equivalents .