A light source device having a light guide (3) for guiding light emitted from a primary light source and emitting light obliquely from a light emission face (33) thereof, and a light deflecting device (4) disposed so as to be adjacent to the light emission face. Plural elongated prisms are arranged in parallel to one another at a light incident surface (41) of the light deflecting device. Virtual elongated prisms (I) each having a vertical angle of 50 to 70 arranged in the same arrangement pitch as the elongated prisms are assumed so that the peak emission light in the light emission distribution of light emitted from the light emission face is incident on one virtual prism face of the virtual elongated prism, internally totally reflected by another virtual prism face and then emitted in a desired direction from a light emission surface (42) of the light deflecting device. A prism face of each elongated prism at the far side from the primary light source is designed to have a convex surface shape with respect to the shape of the virtual elongated prism. The value (r/P) of the radius of curvature r normalized by the arrangement pitch P is equal to 7 to 30, and the ratio (d/P) of the maximum distance between the convex-surface-shaped prism face of the elongated prism and the virtual prism face to the arrangement pitch P is equal to 0.05 to 5%.