PURPOSE: To improve the accuracy of positioning by calculating the moving quantity for moving of a laser beam processing head to a processing start point from the coordinate value of the central axis of the processing head and the coordinate value at the start point of processing set on a work.

CONSTITUTION: The laser beam is projected to the position determined as the reference coordinate point 3(X0, Y0) of the laser beam processing head. The head 2 is then moved in X-Y axis directions until the optical axis of a magnifier 3 mounted to the head 2 and the projection point 4 align to each other. A relative position is determined from the X-Y coordinate value 4'(X1, Y1) of the head 2 when the optical axis of the magnifier 3 and the point 4 are aligned by such movement and said position is stored in a control part. 4 of X-direction reference points (X3, -X3'), (X4, -X4') is determined on the work 1 and the centers between the respective two points are calculated and stored. The lines running the intermediate points in the X-axis and Y-axis direction are designated as L1, L2 and the orthogonal intersection point P0(X2, Y2) thereof is calculated and is determined as the processing start point. The movement of the head is determined by ΔX=X2-X1, ΔY=Y2-Y1.