PURPOSE: To enable detection at any point eliminating mechanical positioning of both the points by determining the difference between the real reference point and the ideal reference point to obtain the coordinate of the actual point to be detection correcting the virtual coordinate of the point being detected with respect to the ideal reference point by the difference.

CONSTITUTION: After a board 1 is retained on a jig 3, as through holes 11A and 11B are irradiated with light sources 10A and 10B, the real reference points are made up by lights passing through the holes. When a command is inputted from a computer 12, moving bodies 5, 7A and 7B are moved under control with controllers 16A and 16B while photosensors 9A and 9B carried on the moving bodies 7A and 7B search real reference points made up by transmission lights from the through holes 11A and 11B. The computer 12 provides a command to the controllers 16A and 16B to shift the moving bodies so that the quantity of light received by four divided photosensors equals to search the reference points correctly.