A light source device includes: a light source lamp having a light-emitting tube with a discharge space and a pair of electrodes disposed in the discharge space of the light-emitting tube; a reflector extending in a substantially concave shape in section, the reflector reflecting a light beam irradiated from the light source lamp; and a sub-reflection mirror having a reflection surface that is disposed to face a reflection surface of the reflector, the sub-reflection mirror reflecting a part of the light beam irradiated from the light source lamp toward the discharge space. The light-emitting tube has a light-emitting section having the discharge space therein and sealing sections provided on both sides of the light-emitting section. The sub-reflection mirror is formed in a shape of a bowl that covers the light-emitting section of the light-emitting tube, the sub-reflection mirror having an opening for mounting the sub-reflection mirror to the light-emitting tube by allowing one of the sealing sections of the light-emitting tube to be inserted thereinto. A heat insulating member is provided to at least a part of the light-emitting section and the other one of the sealing sections of the light-emitting tube.