Abstract: Alight source device includes a resonator having first and second mirrors, a gain medium disposed between the first and second mirrors and including a first semiconductor portion, an active layer, and a second semiconductor portion arranged in this order in a direction perpendicular to an optical axis of the resonator, and having first and second principal surfaces respectively located on sides of the first and second semiconductor portions opposite to sides on which the active layer is provided, a first heat dissipation member located on a first principal surface side of the gain medium, and a second heat dissipation member located on a second principal surface side of the gain medium. The resonator and the gain medium are arranged such that the optical axis passes through the gain medium.