A light source apparatus includes a light source, a detection unit configured to detect light emitted by the light source, a determination unit configured to determine a target brightness, and a control unit configured to periodically perform control of light emission from the light source, wherein the control unit divides one period of the control into a first period during which the light source is caused to emit light that is to be detected by the detection unit and a second period during which the light source is caused to emit light at the target brightness, and during the first period, drives the light source using a driving signal with a value smaller than a value during the second period, and the detection unit detects light emitted by the light source during the first period.