PURPOSE: To improve efficiency without damaging other parts by aligning the upper retainer of a pair of upper and lower retainers with the top surface of a workpiece and fixing it and holding the bottom surface part of said workpiece by means of said lower retainer with the upper retainer as the reference, and on-off controlling these operations.

CONSTITUTION: A lens system member 1 is inserted between the frame upper part 8 and the frame lower part 9 of an assembly jig 5, air is fed to an air supply port 16 and, by releasing air to the atmosphere through this port 16, a retainer 11 is allowed to fall naturally by its own weight, comming in contact with the top surface part of the member 1. After that, air is fed from an air supply port 21 to a cylinder chamber 18 causing a piston 19 to move to make a lock pin 17 hit against a rod part 12, fixing the retainer 11. Then, by feeding air from an air supply port 27 into a cylinder chamber 25, a piston 24 is pushed up, thrusting up a retainer 22, which is pushed against the bottom surface part of the member 1, thereby holding the member 1 between the retainers 11, 12. Then, by operating a registering stage which holds a photosensor 2, the photosensor 2 is bonded to the end face of the member 1 with the centers of optical axes being aligned. When control is performed by means of on-off operation instead of air pressure, the efficiency can be further increased.