Provided are a light source device that can achieve higher accuracy while maintaining reliability by reducing stray light, and a photoelectric encoder including the same. A light source device 2 includes a translucent member 10 configured to accommodate a light emitting unit 5 thereinside, and to make light emitted from the light emitting unit 5, into parallel light. The translucent member 10 includes an optical axis plane 11, an orthogonal plane 12, a reflection portion configured to make light from the light emitting unit 5, into parallel light, and a connection portion 14 formed by the optical axis plane 11 and the orthogonal plane 12 connecting to each other, and having a curved surface. A central axis AX of the curved surface is in an outside region of the translucent member 10, and a region between the optical axis plane 11 and the orthogonal plane 12.