Top surfaces and inclined surfaces of prisms on a side opposite to a light source are formed such that, in a region on a light source side, light traveling in a light guide body toward the side opposite to the light source is sequentially reflected by the top surfaces and the inclined surfaces of the side opposite to the light surface of the prisms and approaches a direction orthogonal to the longitudinal direction of the light guide body. The height of the prisms can be the lowest in the longitudinally central portion of the light guide body, and gradually increases toward the light source side and the side opposite to the light source, from the central portion.