In a socket portion of an endoscopic light source connector, a rod end holder is mounted on a riser plate within a housing of a light source for holding and supporting a fore end portion of a light guide rod of a light source connector, which has a proximal end portion of a light guide encased in a rigid pipe. The rod end holder is constituted by a fixed tubular casing which is mounted on the riser plate in an aligned position relative to an optical axis of illumination light path from a source lamp of the light source, and a slide member which is axially slidably fitted in the fixed tubular casing and internally provided with a light guide passage for passing illumination light from the source lamp. The light guide passage in the slide member is spread continuously or stepwise in the outward direction or toward the front side of the light source to permit connection of light guide rods of different diameters, and the slide member is constantly in an outward direction or in a direction inverse to the direction of insertion of a light guide rod.