A light source apparatus includes a light source, a light source enclosure that accommodates the light source and has a plurality of openings formed therein in a circumferential direction around an optical axis of the light source, a channel having an introduction port through which air is introduced from outside of the light source enclosure, the channel allowed to communicate with the plurality of openings, and an opening/closing mechanism that rotates around the optical axis to selectively open and close the plurality of openings, and the opening/closing mechanism has an opening/closing section that opens and closes the plurality of openings in a first posture in which the optical axis extends along a horizontal plane in such a way that upper openings located on a vertically upper side are open and at least part of lower openings located on a vertically lower side is closed.