PURPOSE: To enable to diaphragm laser light to a small spot and to improve working speed with a device for changing the direction of the laser light with a turn-back mirror and irradiating the laser light to a work via a focusing lens by forming the turn-back mirror with a concave mirror.

CONSTITUTION: A spherical mirror 10 and a condenser lens 5 are disposed between a laser oscillator 1 and a work. The mirror 10 is formed under the condition R>f so that the focal position O1 (focal length is given by R/2) by the average radius R of curvature of the mirror is made larger than the focal position O2 (focal length f) by the lens 5. the lens 5 is disposed in the position where the parallel light of the incident beam and the divergent light are condensed at the same beam diameter D by the mirror 10. The laser beam is condensed without receiving the influence of the spot diameter by the divergent light as in the prior art according to the above-mentioned device and therefore the laser light is diaphragmed to a small spot as shown in the equation.