a first step 7 b 2 is formed in a such shape as to house or dispose an end of the reflection sheet 3 having a rectangular shape and extends from a lower surface 7 a 0 of the white plastic part 7 a to a lower surface 7 b 0 of the black plastic part 7 b to provide a bottom of a hollow together with a lower surface of the light guide 2 to house the reflection sheet 3 in the hollow on the other hand in the white plastic part 7 a a lower inner side wall 7 a 2 see fig2 b faces the light guide plate 2 and has a second step 7 a 3 to house ends of a pair of the prism sheets 5 in addition on the white plastic part 7 a the light shielding sheet 8 see fig1 b is disposed and a third step 7 b 4 is formed in the black plastic part 7 b to house dispose an end of the light shielding sheet 8 on the black plastic part 7 b an outer upper part of the black plastic part 7 b is chamfered to have a chamfered surface 7 b 5 as described above the joint surface 7 a 1 of the white plastic part 7 a and the joint surface 7 b 1 of the black plastic part 7 b.