hereinafter , embodiments of the present invention are described with reference to the drawings . fig1 is an exploded perspective view of an example of a liquid crystal display device that includes a backlight unit according to the present invention . as shown in fig1 , a liquid crystal display device a includes : a liquid crystal display panel 1 ; and a backlight unit 2 that is disposed on a rear surface of the liquid crystal display panel 1 . the liquid crystal display panel 1 has : a liquid crystal panel 11 in which liquid crystal is injected ; a light polarization plate 12 that is attached to a front surface ( viewer side ) of the liquid crystal panel 11 ; and a light polarization plate 13 that is attached to a rear surface ( backlight unit 2 side ) of the liquid crystal panel 11 . the liquid crystal panel 11 includes : an array substrate that includes switching devices such as a tft ( thin film transistor ) and the like ; an opposite substrate that is disposed to oppose the array substrate and is provided with a color filter ; and liquid crystal that is injected between the array substrate and the opposite substrate ( none of them are shown ). here , the liquid crystal panel 1 may have a structure in which the color filter is formed on the array substrate and a transparent electrode is formed on the opposite substrate . as shown in fig1 , in the liquid crystal display device a , an optical sheet unit 3 is disposed between the liquid crystal display panel 1 and the backlight unit 2 . in the liquid crystal display device a shown in fig1 , the optical sheet unit 3 includes three optical sheets , which however is not limiting , and may include more sheets , or may be composed of a smaller number of sheets . the optical sheet unit 3 includes , for example , a diffusion sheet , a lens sheet and the like . the diffusion sheet diffuses input light inside . in other words , the light entering the diffusion sheet is diffused in the diffusion sheet and is output as planar light that has less brightness unevenness . the lens sheet is a sheet that deflects ( collects light ) a light radiation characteristic ; entering light is collected , whereby light emission brightness per unit area improves . here , there is a case where an optical sheet other than these is used . the light output from the backlight unit 2 enters the optical sheet unit 3 . the light entering the optical sheet unit 3 is diffused and collected , whereby the brightness unevenness is reduced , the light emission brightness is raised , and the light enters the liquid crystal display panel 1 . the backlight unit 2 is described in detail with reference to drawings . fig2 is an exploded perspective view of a backlight unit used in the liquid crystal display device shown in fig1 , and fig3 is a rear view of a light guide plate that is used in the backlight unit shown in fig2 . the backlight unit 2 is a backlight unit of edge - light type . as shown in fig2 , the backlight unit 2 includes : a backlight chassis 4 that is a chassis ; a light guide plate 5 that is held in the backlight chassis 4 ; and a plurality of led lamps 6 ( here , five each on the left and right , that is , ten in total ) that are light sources held in the backlight chassis 4 and output light to the light guide plate 5 . as shown in fig2 , the backlight chassis 4 includes a rectangular rear surface portion 41 and a side wall portions 42 that are formed to rise from four edges of the rear surface portion 41 . a reflection sheet ( not shown ) is mounted to cover the rear surface portion 41 and side wall portions 42 of the backlight chassis 4 . the reflection sheet is intended to reflect and return light leaking from the light guide plate 5 back to the light guide plate 5 , and usually , a white sheet is used . it is preferable that the reflection sheet is fixed to the backlight chassis 4 not to deviate upward . because of this , as the fixing method for the reflection sheet , for example , there is a method for bonding a rear surface of the reflection sheet to the rear surface portion 41 and the side wall portions 42 by means of a double - adhesion type adhesive sheet . the light guide plate 5 is a transparent rectangular parallelepiped plate - shaped member . the light guide plate 5 is provided , on side surfaces , with light reception portions 51 that receive the light from the led lamp 6 . here , in the light guide plate 5 , both end surfaces in a long - edge direction are the light reception portions 51 . besides , one of main surfaces of the light guide plate 5 defines an output portion 52 from which the light input from the light reception portion 51 exits . besides , a surface opposite to the output portion 52 defines a mounted portion 53 that opposes the rear surface portion 41 of the backlight chassis 4 . the light guide plate 5 and the led lamp 6 are mounted in the backlight chassis 4 where the reflection sheet is disposed . as shown in fig3 , the mounted portion 53 of the light guide plate 5 is provided with leg members 531 . the leg member 531 is a cylindrical member that meets the mounted portion 53 having a rectangular flat surface at right angles . besides , the leg members 531 are disposed at four corners and at intermediate portions of respective edges of the rectangular mounted portion 53 . on the other hand , the rear surface portion 41 of the backlight chassis 4 is provided with hold members 411 , that is , through - holes , at positions which correspond to the leg members 531 when the light guide plate 5 is disposed in the backlight chassis 4 . besides , the reflection sheet also is provided with a through - hole which the leg member 531 penetrates . the leg member 531 penetrates the through - hole of the reflection sheet and the holding portion 411 of the rear surface portion 41 , whereby the light guide plate 5 is positioned at a proper position of the backlight chassis 4 . besides , the leg member 531 is inserted into the hold member 411 , accordingly , the light guide plate 5 is unlikely to deviate . according to this , it is possible to reduce fixing members such as an adhesive , an adhesive sheet and the like that are intended to fix the light guide plate 5 to the backlight chassis 4 . here , it is preferable that the leg member 531 has enough strength such that the leg member 531 is not deformed nor broken even if force acts on the light guide plate 5 . the leg member 531 may be formed integrally with the light guide plate 5 or may be detachably mounted on the light guide plate 5 . besides , the leg member 531 is a cylindrical member , but may be a pillar member that is tapered to become slender toward its tip end . by tapering the leg member 531 , it is possible to insert the leg member 531 into the holding portion 411 without performing fine adjustment . according to this , it is possible to easily mount the light guide plate 5 in the backlight chassis 4 . in the above backlight unit 2 , the light guide plate 5 includes the eight leg members 531 ; however , this is not limiting , and may include more or less than eight in accordance with the size of the light guide plate 5 if it is possible to stably dispose the light guide plate 5 in the backlight chassis 4 . another example of the backlight unit according to the present invention is described with reference to a drawing . fig4 is a schematic view of another example of a light guide plate and a backlight chassis of the backlight unit according to the present invention . a light guide plate 5 a shown in fig4 has the same structure as the light guide plate 5 shown in fig3 except that the position of a leg member 531 a is different . besides , a backlight chassis 4 a has the same structure as the backlight chassis 4 shown in fig2 except that the position of a holding portion 411 a is different . here , in fig4 , the substantially same portions are indicated by the same reference numbers , and detailed description of the same portions is skipped . besides , for comparison with the light guide plate 5 shown in fig3 , the leg member 531 is indicated by means of a two - dot - one - bar line . besides , a corresponding relationship between the leg member 531 a and the holding portion 411 a is indicated by means of a one - dot - one - bar line . as shown in fig4 , the leg members 531 a are disposed near the four corners and near the central areas of the respective edges of the rectangular mounted portion 53 . however , unlike the light guide plate 5 , the eight leg members 531 a are disposed such that the positions of the respective leg members 531 a do not become point - symmetrical with respect to the center of the mounted portion 53 . and , the rear surface portion 41 of the backlight chassis 4 is provided with the holding portion 411 a at the position that corresponds to the leg member 531 a when the direction of the light guide plate 5 a is a correct mount direction . when the direction of the light guide plate 5 a is correct with respect to the backlight chassis 4 a , the leg member 531 a is inserted into the holding portion 411 a , and the light guide plate 5 a is positioned and mounted in the backlight chassis 4 a . in a case where the light guide plate 5 a rotates 180 ° about an axis that passes through the center of the mounted portion 53 and meets the mounted portion 53 at right angles , the leg member 531 a and the holding portion 411 a do not overlie each other when viewed from front , accordingly , the leg member 531 a is not insertable into the holding portion 411 a . in other words , the light guide plate 5 a is mountable in the backlight chassis 4 a in the correct direction only . according to this , without strictly checking the direction of the light guide plate 5 a , the light guide plate 5 a is mountable in the backlight chassis 4 a in the correct direction . still another example of the backlight unit according to the present invention is described with reference to a drawing . fig5 is a schematic view of another example of a light guide plate and a backlight chassis of the backlight unit . a light guide plate 5 b shown in fig5 has the same structure as the light guide plate 5 shown in fig3 except that the light guide plate 5 b includes a leg member 531 b . besides , a backlight chassis 4 b has the same structure as the backlight chassis 4 shown in fig2 except that the shape of a holding portion 411 b of the backlight chassis 4 b is different . the substantially same portions are indicated by the same reference numbers , and detailed description of the same portions is skipped . as shown in fig5 , the leg members 531 b are disposed at the four corners and in the central areas of the respective edges of the mounted portion 53 . here , as described above , the position of the leg member 531 b is the same as the leg member 531 of the light guide plate 5 . and , a section shape cut along the plane parallel to the mounted portion 53 of the leg member 531 b is a regular pentagon . besides , the rear surface portion 41 of the backlight chassis 4 b is provided , at the position corresponding to the leg member 531 b of the light guide plate 5 b , with the holding portion 411 b that has the same regular pentagon as the sectional shape of the leg member 531 b . only when the light guide plate 5 b is in the correct direction with respect to the backlight chassis 4 b , the leg member 531 b is inserted into the holding portion 411 b , and the light guide plate 5 b is positioned and held in the backlight chassis 4 b with high accuracy . in a case where the light guide plate 5 b rotates 180 ° about the axis that passes through the center of the mounted portion 53 and meets the mounted portion 53 at right angles , the leg members 531 b each move to positions that are symmetrical with the center of the mounted portion 53 . at this time , the sectional shape of the leg member 531 b is the regular pentagon and is not point - symmetrical . and , the shapes of the leg member 531 b and the holding portion 411 b are rotated 180 ° from each other to be in a reverse relationship with each other . accordingly , the leg member 531 b is not insertable into the holding portion 411 b . in other words , the light guide plate 5 b is mountable in the backlight chassis 4 b in the correct direction only . according to this , without strictly checking the direction of the light guide plate 5 b , the light guide plate 5 b is mountable in the backlight chassis 4 b in the correct direction . here , the example is described , in which in the above backlight unit , the sectional shape of the leg member 531 b and the shape of the holding portion 411 b are the regular pentagons ; however , this is not limiting , and a shape , which is a triangle , a fan shape and the like that are not point - symmetrical ( do not match each other when the light guide plate is rotated 180 ° as described above ), may be widely usable . besides , in the above backlight unit , the sectional shapes of all the leg members 531 b and the shape of the holding portion 411 b are the regular pentagons ; however , this is not limiting , and even if the shape of at least one set of the leg member positions that are symmetrical with respect to the center of mounted portion 53 and the shape of the holding portion 411 b corresponding to this leg member are shapes that are not point - symmetrical , the same effects are obtainable . in other words , without strictly checking the light guide plate , the light guide plate is mountable in the backlight chassis in the correct direction . still another example of the backlight unit according to the present invention is described with reference to a drawing . fig6 is a schematic view of another example of a light guide plate and a backlight chassis of the backlight unit . a light guide plate 5 c shown in fig6 has the same structure as the light guide plate 5 shown in fig3 except that one of the eight leg members 531 is a leg member 531 c . besides , a backlight chassis 4 c has the same structure as the backlight chassis 4 shown in fig2 except that the shape of a holding portion 411 c of the backlight chassis 4 c is different . the substantially same portions are indicated by the same reference numbers , and detailed description of the same portions is skipped . as shown in fig6 , the light guide plate 5 c includes the leg member 531 c in an upper right area of the mounted portion 53 when viewed from rear . and , the leg members 531 are disposed at the remaining corners and in the central areas of the respective edges of the mounted portion 53 . here , as described above , the positions of the leg member 531 c and the leg member 531 are the same as the leg member 531 of the light guide plate 5 . and , the rear surface portion 41 of the backlight chassis 4 c is provided , in an upper left area when viewed from front , with the holding portion 411 c . besides , at the positions corresponding to the remaining leg members 531 , the same holding portions 411 as in the backlight chassis 4 are formed . the sectional shape of the leg member 531 c cut along the plane parallel to the mounted portion 53 is a square . likewise , the shape of the hold member 411 c also is a square . as shown in fig6 , when the direction of the light guide plate 5 c is correct with respect to the backlight chassis 4 c , the leg member 531 c corresponds to the holding portion 411 c , and the light guide plate 5 c is positioned and held in the backlight chassis 4 c with high accuracy . in a case where the light guide plate 5 c rotates 180 ° about the axis that passes through the center of the mounted portion 53 and meets the mounted portion 53 at right angles , the leg member 531 c opposes the holding portion 411 at a lower right portion when viewed from front . at this time , the sectional shape of the leg member 531 c is the square and the shape of the holding portion 411 at the lower right portion is the circle , accordingly , the leg member 531 c is not insertable into the holding portion 411 . likewise , the leg member 531 , which opposes the holding portion 411 c because of the rotation , has the circular sectional shape that is a shape different from the square holding portion 411 c , accordingly , is not insertable . in other words , the light guide plate 5 c is positioned and held in the backlight chassis 4 c in the correct direction only with high accuracy . according to this , without strictly checking the direction of the light guide plate 5 c , the light guide plate 5 c is mountable in the backlight chassis 4 c in the correct direction . here , the sectional shape of the leg member 531 is the circle and the sectional shape of the leg member 531 c is the square , accordingly , when the sectional shape of the leg member 531 is larger than the sectional shape of the leg member 531 c , the leg member 531 c is insertable into the holding portion 411 . even in this case , the leg member 531 is not insertable into the holding portion 411 c . in a case where the section of the leg member 531 is smaller than the section of the leg member 531 c , reversely , even if the leg member 531 is insertable into the holding portion 411 c , the leg member 531 c is not insertable into the holding portion 411 . in other words , irrespective of the size relationship between the leg member 531 c and the leg member 531 , the light guide plate 5 c is mountable in the backlight chassis 4 c in the correct direction . here , in the above description , the example is described , which includes one leg member that has the different sectional shape ; however , this is not limiting , and the light guide plate may have a structure that includes two or more kinds of leg members whose sectional shapes are different from one another . at this time , of the sets of the leg members that are disposed symmetrically with respect to the center of the mounted portion , at least one set is a set of leg members that has sectional shapes different from each other . further , in the above example , to prevent the leg member from being inserted into the holding portion when the light guide plate rotates , the sectional shape of the leg member is modified ; however , the sectional shape may be the same , while the sectional size may be modified . in this case as well , the leg member having a large sectional shape is not insertable into the holding portion into which the leg member having a small sectional shape is inserted . according to this , only when the light guide plate is in the correct direction , the light guide plate is positioned and held in the backlight chassis with high accuracy . still another example of the backlight unit according to the present invention is described with reference to a drawing . fig7 is a schematic view of another example of a light guide plate and a backlight chassis of the backlight unit . a light guide plate 7 shown in fig7 includes a plurality of light guide members 70 ( here , five ; however , this is not limiting ), and the light guide members 70 are ranged and disposed in the backlight chassis 4 . the light guide members 70 are a transparent rectangular parallelepiped plate - shaped member and ranged in a short - edge direction to compose the light guide plate 7 . in the light guide member 70 , a side surface defines a light reception portion 701 that receives the light from the led lamp 6 . here , in the light guide member 70 , both end surfaces in a long - edge direction are the light reception portions 701 . besides , one of main surfaces of the light guide member 70 defines an output portion from which the light received by light reception member 701 exits . besides , a surface opposite to the output portion defines a mounted portion 703 that opposes the rear surface portion 41 of the backlight chassis 4 . as shown in fig7 , the light guide member 701 is provided , near both end portions in a long - edge direction of the mounted portion 703 , with leg members 704 that protrude in a direction that meets the mounted portion 703 at right angles . the rear surface portion 41 of the backlight chassis 4 is provided , at positions that correspond to the respective leg members 704 , with holding portions 412 , that is , through - holes into which the leg members 704 are inserted when the light guide members 70 are ranged and disposed in the backlight chassis 4 . by inserting the leg members 701 into the holding portions 412 to range and dispose the light guide members 70 , it is possible to easily form the light guide plate 7 in the backlight chassis 4 . besides , by adjusting brightness of the light from the led lamp 6 for every light guide member 70 , it is possible to perform area control in which the light having the brightness different for every region is output . here , by adding the features of the above shape , disposition , size and the like to the leg member 704 and the holding portion 412 , it is possible to dispose the light guide member 70 in the backlight chassis 4 in the correct direction . here , illustration and description are skipped ; however , a reflection sheet is disposed between the light guide member 70 and the backlight chassis 4 . and , the leg member 704 penetrates a through - hole that is formed through the reflection sheet . here , the holding portion of the backlight chassis in each of the above embodiments is the through - hole formed through the rear surface portion 41 ; however , may be a closed hole other than the through - hole . for example , if the backlight chassis is formed of a metal plate , it is possible to produce by means of extrusion molding of the metal plate . in addition , besides this , a method for forming a hole into which the leg member is smoothly insertable may be used . in the backlight unit described in each of the above embodiments , the light guide plate ( or the light guide member ) has the rectangular parallelepiped shape , and the leg member , which is the engagement structure engaging with the backlight chassis of the light guide plate , has the shape that protrudes outside the light guide plate , and the light from the light source is not blocked by the engagement structure and is not unevenly reflected ( irregular reflection ) at the portion where the cutout and the like are formed , accordingly , it is possible to alleviate the brightness unevenness of the planar light output from the output portion . besides , by inserting the leg member into the holding portion , it is possible to position the light guide plate in the backlight chassis with high accuracy , accordingly , it is possible to output the planar light from the output portion to the region of the liquid crystal display device where the image display is performed . besides , the light guide plate is mountable in the backlight chassis in the correct direction , accordingly , the light reception portion is disposed correctly to oppose the light source , and it is possible to receive the light from the light source by means of the light reception portion with no loss . according to this , it is possible to increase the brightness of the planar light output from the backlight unit . further , in the above embodiments , the equal number of the led lamps , that is , the light sources , are disposed at the left and right of the light guide plate ; however , because of restriction on the space and a device mounted on the backlight unit , there is a case where it is impossible to perform the disposition with good balance . in this case as well , also , the light guide plate has the structure which includes the leg member that protrudes from the rectangular parallelepiped , accordingly , the structure is simple and the production is easy . besides , the light guide plate has the simple structure , accordingly , the optimum designing for efficiently transmitting the light in the light guide plate is easy . besides , it is possible to alleviate the light guide plate rotating and the light guide plate being mounted in the backlight chassis in a different direction . according to this , it is possible to easily demonstrate the performance of the optimized light guide plate . the present invention is usable as a display device of apparatuses such as a flat - screen television device , a flat - screen display device , a mobile phone and the like .