A diffusion plate (DL) allowing the light from a fluorescent tube (71) to transmit therethrough is held on a support surface (1S, 1S) forming one surface of a side holder (SF). In particular, a support frame (FM) which also plays the role of the side holder (SF) supports the diffusion plate (DL) of a shape of which the long dimension and short dimension parts can be defined, for example, a rectangular shape at only two short dimension parts on its outer edges opposed to each other.