A light source apparatus includes: a lamp, a diaphragm device, a CPU that causes the diaphragm device to execute adjustment of an emission amount of illumination light on the basis of a target brightness signal generated based on brightness of an image obtained by picking up an image of a subject a CPU that switches between illumination modes, and a mode switching switch. During a period after an instruction for switching from one of an illumination mode corresponding to normal light observation mode and an illumination mode corresponding to special light observation mode to the other is given until the switching by the CPU is completed, the CPU causes the diaphragm device to execute adjustment of the emission amount of the illumination light based on a correction value obtained by correcting a current diaphragm value at the time when the instruction for switching is given using a predetermined coefficient.
