PURPOSE: To correct positional shift of marking on a work through simple operation.

CONSTITUTION: An image of a work 11 viewed by means of a CCD camera 12 through first and second scanner mirrors 14, 15, and a cross line based on a cross line signal generated from a digital electronic line generator 19 are displayed on a CRT 18. The work 11 is previously provided with a plurality of reference points so that the cross line is aligned with the irradiating position of the work 11 at all times and then data is inputted on a keyboard 24 so that the cross line is aligned with each of the plurality of reference points. A computor 23 corrects the position to be scanned by means of a laser beam emitted from a laser oscillator 25 based on the data thus inputted.