A light source apparatus for an electronic endoscope includes a light source; a rotary shutter including a pair of aperture controlling rotary plates; a first planetary gear mechanism including a first internal tooth gear, a first sun gear rotated with one aperture controlling rotary plate, and a first planet gear engaged with the first internal tooth gear and the first sun gear; a second planetary gear mechanism including a second internal tooth gear, a second sun gear rotated with the other aperture controlling rotary plate, and a second planet gear engaged with the second internal tooth gear and the second sun gear; and a carrier device which holds the first and second planet gears. An endless timing belt is installed on an output gear of a phase-difference motor and an input tooth portion of the second internal tooth gear, and includes a meshing portion.