To smooth the roughness of a cutting plane without decreasing a cutting speed at the time of carrying out laser cutting a piece to be worked by using a laser beam as a heat source.

A curvature varying mirror 1 for controlling the angle of the dispersion of a laser beam by elastically deforming a mirror plate 1a by a piezoactuator 1c is mounted on a laser working machine, and the laser cutting is carried out by vibrating the focal position of a laser beam 4 irradiating a piece to be worked from the laser working machine to the optical axial direction. Also, the focal vibration at that time satisfies the condition that frequency ranges from 150 Hz to 300 Hz and vibration width ranges from ±0.5 mm to ±3 mm.