A light source device includes a casing including one end surface, other end surface, and one side surface between the one end surface and the other end surface, a light emitting unit provided on a side of the one end surface in the casing, a heat radiating unit provided in the casing and thermally connected to the light emitting unit, an exhaust port formed at least in a region located in one side surface and facing the heat radiating unit, an axial fan which is provided in the casing and supplies cooling air to the heat radiating unit, and a shielding deflecting plate which is provided between the axial fan and the heat radiating unit and shields a part of the cooling air supplied from the axial fan and deflects a part of the cooling air to a direction away from the exhaust port.