A jig mounting device which comprises a detection optical system for detecting the identification mark of a glasses lens and is constructed so as to determine a mounting point based on an identification mark detected by the detection optical system, position the mounting center of the jig used when a glasses lens is processed onto the mounting point, and mount the jig on the surface of the glasses lens, wherein the detection optical system comprises a condensing optical system for condensing a luminous light flux from a light source onto the surface of the glasses lens formed with a concealed mark via a stop, a reflection plate for reflecting the light flux condensed by the condensing optical system and passed through the glasses lens, and an imaging means provided to be positioned almost optically conjugate with the stop so as to be focused on a spatial portion along the optical axis from the surface of the glasses lens. |