A medical light source device includes: a first light emitting device configured to emit light； an optical member provided on an optical path of the light emitted from the first light emitting device, and configured to emit a part of the light emitted by the first light emitting device in a first direction and emit rest of the light in a second direction different from the first direction； an illumination optical system provided on the optical path of the light emitted in the first direction by the optical member and configured to guide incident light to be emitted to an outside； a detector disposed on the optical path of the light emitted in the second direction by the optical member and configured to detect an amount of incident light； and an optical path changer configured to change the optical path of the light emitted from the first light emitting device in accordance with the amount of light detected by the detector.
