The focal point of a working laser beam is made to follow precisely the laser-beam irradiated face of an object to be worked. A cutting-scheduled line (5) is irradiated along itself with a measuring laser beam. An astigmatism is added to the reflected light reflected on such a surface (3) of the working object (1) as is irradiated with the measuring laser beam. A displacement sensor signal is detected according to the focused image of the reflected light having the astigmatism added. A displacement sensor signal is made to have the feedback reference value according to the light quantity of the reflected light, so that the focal point ofthe working laser beam is adjusted to a predetermined position with respect to the surface (3). Even if an area having an extremely low reflectivity of the measuring laser beam is partially present so that the light quantity of the reflected light of the measuring laser beam drops, the focal point of the working laser beam drops can follow the surface (3) of the working object (1) reliably and precisely.
