A light source device includes: a phosphor rotating body on which a phosphor that is formed on a surface and that is used in a rotating state with an axis of rotation that is perpendicular to the surface; a first excitation light source that generates first fluorescent light by irradiating first excitation light upon a first position that is separated from the center of rotation on the surface; a detection light generation unit that irradiates detection light upon a second position that is separated from the center of rotation on the surface and that differs from the first position; an optical detection unit that supplies a detection signal corresponding to emission light emitted from the phosphor rotating body in response to the detection light; and a control unit that receives the detection signal and controls the intensity of the first excitation light on the basis of the detection signal.