A light source apparatus for an electronic endoscope includes a light source; a rotary shutter including a pair of coaxial aperture controlling rotary plates, which are selectively rotatable relative to and together with each other and each includes light interception portions and opening portions, wherein a combined opening angle of the opening portions is varied by a relative rotation of the aperture controlling rotary plates, and the amount of emitted light is controlled by varying the combined opening angle, and the controlled amount of the light is emitted through the rotary shutter by integral rotation of the aperture controlling rotary plates; and a mechanical stop device, positioned between the aperture controlling rotary plates, for limiting the range of relative rotation angle between the aperture controlling rotary plates from a maximum opening angle to a minimum opening angle.