A light source apparatus 3 includes: a lamp 23 a diaphragm device 25 a CPU 41c that causes the diaphragm device 25 to execute adjustment of an emission amount of illumination light on the basis of a target brightness signal CS generated based on brightness of an image obtained by picking up an image of a subject a CPU 41a that switches between illumination modes and a mode switching switch 22a. 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 41a is completed, the CPU 41 c causes the diaphragm device 25 to execute adjustment of the emission amount of the illumination light based on a correction value DA obtained by correcting a current diaphragm value DC at the time when the instruction for switching is given using a predetermined coefficient α, β.
