PURPOSE: To execute the work cutting whose cutting quality is satisfactory by preparing plural lens units being freely attachable and detachable to and from a working head, and installing the lens unit having optimal depth of focus and the incident beam diameter for obtaining the spot diameter in accordance with plate thickness of the work.

CONSTITUTION: Plural freely attachable and detachable lens units 3 provided on a working head 1 are provided with a condenser lens 33 for obtaining optimal depth of focus in accordance with plate shickness of a work 2 to be cut, and lenses 31, 32 for obtaining the incident beam diameter by which the condensing spot diameter by its condenser lens 33 becomes minimum. In such a way, by selecting the lens unit 3 in accordance with plate thickness of the work 2 to be cut and attaching it to the working head 1, the work 2 is cut.