A light source device includes a light source section that emits laser beam, a member reducing light quantity unevenness of the laser beam, a diffusion member that is provided in a traveling optical path of the laser beam between the light source section and the member reducing light quantity unevenness and diffuses the laser beam, a detection element that detects a physical characteristic of the diffusion member and outputs a state detecting signal, and an abnormal state determiner that determines an abnormal state of the diffusion member based on the state detecting signal.