An optical deflector element has a light incoming surface (41) in which light enters and a light outgoing surface (42) located opposite the light incoming surface and from which light exits. In the light incoming surface (41) are arranged elongated prisms (41A) in parallel to each other. Each of the elongated prisms (41A) is formed by a top end flat face (46) located at the top end of the elongated prism and having an inclination angle (b) of 1 to 50 degrees, a first prism face (44) located at one side of the flat face, and a second prism face (45) at the other side. A light source device is formed by combining the optical deflector element, light guide and a primary light source. The light guide has a light incident face in which light emitted from the primary light source enters, and a light emitting face from which guided light exits. The combination above provides a light source device that does not have degradation in luminance and has excellent quality free from visual defects including a black stripe and a Moire pattern.