A light source device, comprising a light guide (3) having two light incident surfaces opposed to each other for receiving incident light radiated from a primary light source and a light outgoing surface (33) for emitting the incident light and a light deflection element (4) disposed adjacent to the light emitting surface side of the light guide, the light deflection element (4) further comprising a light receiving surface (41) for receiving incident light and a light emitting surface (42) positioned on the opposite side thereof and emitting the incident light, wherein a plurality of elongated prisms formed of two prism faces are disposed on the light receiving surface (41) generally parallel with each other, the prism surfaces are formed of at least two faces (46, 47, 48, 49, 50, 51) with different inclination angles, and the inclination angles are larger for the faces positioned nearer the light emitting surface (42).