PURPOSE: To simply and accurately detect a lens mounting position by rotating a cap attached with a lens on a V-shaped groove, and discriminating the improper lens mounting position when the image position change on a screen is larger than a predetermined value.

CONSTITUTION: A cap 2 attached with a lens is placed on a stationary base 3 having a V-shaped groove 31, laser rays are irradiated to the lens along the center axis 6 of a cap 2, a parallel laser which passes the lens is focused on an optical screen 4, the cap 2 is rotated to observe whether the image position on the screen is varied or not. If the variation is larger than a predetermined value, the lens mounting position to the cap 2 is judged to be improper. Thus, whether the lens is correctly mounted on the cap or not can be simply and accurately judged nondestructively.