To provide a mounting method capable of excellently mounting a lens member to a case member with sufficient strength by leaser welding.

The case member 1 provided with a barrel holder 2 having an slope 2a in an opening, the lens member 3 configured such that a lens 5 is mounted to a barrel 4 and a wedge ring 6 are prepared, the lens member 3 is inserted into the lens holder 2 and the lens 5 is focused. The wedge ring 6 is fit to the barrel 4, the circumferential slope 6a comes into contact with the slope 2a of the lens holder 2 and after that, the wedge ring 6 is welded to the barrel holder 2 and the barrel 4 by irradiation with a laser L, to mount the lens member 3 to the case member 1. Excellent laser welding can be stably achieved in a focused state and the barrel 4 of the lens member 3 can be mounted to the lens holder 2 of the case member 1 with sufficient strength.