A light source device comprises: a light source; a light-transmissive plate having a side surface or a main surface facing the light source; a holding frame having a frame-shaped surface holding a peripheral edge portion of the light-transmissive plate, the holding frame having a frame shape; and a spacer provided between the peripheral edge portion of the light-transmissive plate and the frame-shaped surface of the holding frame. The holding frame comprises: a first rib provided on the frame-shaped surface, at a position closer to an outer edge of the holding frame than a contact position of the frame-shaped surface in contact with the spacer is, the first rib extending along the frame shape; and a first recessed portion provided between the first rib and the contact position.