To position and assemble optical parts, etc., with good accuracy with simple constitution.

The light passed through an optical element of the second optical parts 6 and an optical element of the first optical parts 4 is detected and the mispositioning of the optical element of the second optical parts 6 and the optical element of the first optical parts 4 is detected. The mounting position of the second optical parts 6 is varied and the second optical parts 6 are aligned and fixed to the first optical parts 4 according to the detected mispositioning.