To easily and positively open an oblong hole by providing a jig with a protruding body on the lens contact side so as to be inserted in a lens for positioning, and providing a guide hole for guiding a drilling tool in an overlap position with the protruding body.

A protruding body 11 of a jig is inserted in a first hole H1 opened in a lens R. When a drill A is then lowered, the tip of the drill A moves directly below being guided along a guide hole 12. When the tip of the drill A passes through the guide hole 12, the tip of the drill A comes in contact with the bottom face of the protruding body 11. When the drill A is further lowered, a second hole H2 is formed in the lens R while chipping part of the protruding body 11. At the time point of the drill tip going through the lens R, drilling of the second hole is completed so as to form a key-hole communicated with the first hole 1.