PURPOSE: To surely irradiate a laser beam to a desired position on an object to be machined even when there is deviation in the irradiating position of laser beam to the position of a machining head.

CONSTITUTION: Three machining heads are provided freely movably in the X-axis direction above a transfer route on which a hoop material is fed in the Y-axis direction. A reference block 23 having a groove part 23a in the middle is provided on the right hand of the transfer route. A CCD camera and image processor for detecting the reflected light on the reference block 23 are provided. A distance X1 to the left end part of the groove part 23a and distance X2 to the right end part from the original point position O of the machining head are already known. While outputting the low-power laser beam L for detecting, the machining head is rightward moved from a point A and a position at the moment when the reflected light is eliminated is defined as x1. Next, the head is leftward moved from a point D and a position at the moment when the reflected light is eliminated is defined as x2. The amount δof deviation between the mechanical position of the machining head and the irradiating position of the laser beam in the X-axis direction is δ=(X1+X2)/2-(x1+x2)/2.