PURPOSE: To change focal lengths of a convex lens, a concave lens, and a plane disc, by applying pulses to piezoelectric elements, which are provided on outside circumferences of the convex lens, the concave lens, and the plane disc, to distort them.

CONSTITUTION: Piezoelectric elements 4 are arranged at uniform intervals in the circumferential direction on respective outside circumference of a convex lens 1, a concave lens 2, and a plane disc 3, and rings 5 are provided on the outside of piezoelectric elements 4 to fix piezoelectric elements 4. A pulse voltage is applied to piezoelectric elements 4 to distort them, and the convex lens 1, the concave lens 2, and the plane disc 3 are deformed by this distortion to change their focal lengths. It is desirable that the convex lens 1, the concave lens 2, and the plane disc 3 are formed with materials having a large optical modulation effect such as GaP, PbMoO4, or LiNbO3. Since the change of the focus causes the change of the energy density on a certain plane, it is used to produce a perforating work machine using the laser.