PURPOSE: To prevent a return light of a laser oscillator and to suppress damage of a work by providing a monitor means for a laser output, and also, displaying an alarm and stopping an irradiation of a laser light, when its output value exceeds a reference value.

CONSTITUTION: The title device is provided with a discharge tube 3 for outputting a laser light YA1 to a work 4, and the discharge tube 3 is provided with a power sensor 6 for measuring a monitor use laser output for transmitting through a reflecting mirror 3a and outputting a monitor value PA1. In this state, the sum of an output command value PC1 and a set increment allowable value PO1 is compared with the monitor value PA1 by a comparing means 9a, and when the monitor value PA1 is larger, an abnormality detecting signal AR1 is outputted and an alarm is displayed. Simultaneously, an irradiation of the laser light YA1 is stopped. Since an abnormal reflection from the work 4 is detected instantaneously, a return light of a laser oscillator is prevented, and the generation of damage of the work is suppressed to the minimum.