A light source device includes a mounted substrate which is a multi-layered substrate, a semiconductor light-emitting device which emits a laser beam, a wavelength-converting member which radiates fluorescence by being irradiated with the laser beam emitted from the semiconductor light-emitting device as an excitation light, a state detection circuit, an electric field effect type transistor which adjusts an electric current amount applied to the semiconductor light-emitting device upon receipt of an output from the state detection circuit, and an external connecting member, and the semiconductor light-emitting device, the state detection circuit, the transistor, and the external connecting member are mounted on the single mounted substrate.