PURPOSE:To correctly detect a reduction in the energy of a laser beam by a method wherein the peak value and power value of a laser beam ate detected. CONSTITUTION:A laser beam L1 emitted by a laser oscillating device 2 is so controlled as to be steady at a prescribed value through the regulation of an attenuator by means of an attenuator controlling signal S4 supplied by a controlling section 17. In the path of the laser beam L1, beam splitters 4 and 5 are installed, whereby the laser beam L1 is split into laser beams L3 and L4. The beams L3 and L4 are led to a peak value detector 13 and power value detector 14, respectively. The detector 13 detects the peak value of the laser beam L3 for one laser making process and feeds the collected data into the controlling section 17. The detector 14 detects the average power value of the laser beam L4 and feeds the collected data into the controlling section 17.