A light source apparatus for an endoscope having a light source for emitting a bundle of illuminating light rays which are to be supplied to a light guide of the endoscope. The apparatus includes at least a pair of light-blocking plates disposed in the bundle of illuminating light rays to face each other across the optical axis of the ray bundle, and a device for supporting the light-blocking plates such that the plates are rotatable about an axis extending therebetween at right angles to the optical axis. At least one of the light-blocking plates has such a cross-sectional configuration as viewed in the optical axis direction that the light-blocking plate is bent at an angle to the axis of rotation near the center of the illuminating optical path.