An NC laser for metal working which consists of a combination of a laser oscillator and a numerical control device. Abnormal conditions of the device at rest can be detected prior to starting the machining. At the time of start, an instruction for a laser output (Pc) is sent to an output control circuit (2), and the laser output is monitored by a power sensor (2e) to find a ratio of an instruction value (Pc) to a monitor value (Pa). The ratio is stored in a non-volatile memory (10) to find a difference between this ratio and the ratio found at the time of previous starting. When the difference is greater than a preset value, an alarm is displayed on a display screen. When there is a difference greater than a predetermined value between the ratio of this time and the ratio of the previous time, it is regarded that some abnormal condition has developed during the stop period and an alarm is produced.