PURPOSE: To increase the quantity of a product by detecting the abnormal a laser, and to improve the operating efficiency by specifying a fault position. CONSTITUTION: When the oscillation (b) of a laser beam L is detected by laser oscillation detecting means 11 if a laser oscillation command (a) is not detected by laser oscillation command detecting means 10, an abnormality signal S1 representing that a laser is abnormal is output. When the oscillation (b) of the beam L is not detected by the mean 11 at the time when the time from the detection of the command (a) reaches a set time, an abnormality signal representing the abnormal laser is output. The fault position (a thyratron or a charging circuit) of the laser is judged based on the detected results of the first and second detecting means.