A light source apparatus for an electronic endoscope includes a light source; a rotary shutter having a pair of aperture controlling rotary plates; a first planetary gear mechanism including a first internal tooth gear, a first sun gear, and a first planet gear; a second planetary gear mechanism including a second internal tooth gear, a second sun gear, and a second planet gear; and carriers holding the first and second planet gears in a same phase position and supporting the first and second planet gears. One of the first sun gear and the first internal tooth gear is non-rotatably fixed, and the other thereof is rotated together with one of the aperture controlling rotary plates, and one of the second sun gear and the second internal tooth gear of the second planetary gear mechanism is driven together with the other thereof by a phase difference motor.