The present invention realizes a light source unit and a method of cooling the light source unit that reduces any increase in the ambient temperature of a red solid-state light source and thus reduces any decrease in brightness of the red solid-state light source. The light source unit has: a wall having a window, the wall being provided between a light tunnel-side circulation unit that accommodates a light tunnel that is irradiated by the output light of a solid-state light source and a phosphor wheel-side circulation unit that accommodates a phosphor wheel that is excited by the emitted light of the light tunnel; a light tunnel-side heat-receiving heat sink that is provided in the light tunnel-side circulation unit; a phosphor wheel-side heat-receiving heat sink provided in the phosphor wheel-side circulation unit, a heat-radiating heat sink that is connected to the light tunnel-side heat-receiving heat sink and to the phosphor wheel-side heat-receiving heat sink by means of a heat pipe; and a cooling fan that cools the heat-radiating heat sink.