An NC laser for metal working which has the function of indicating the maintenance information on optical parts, and which consists of a combination of a numerical control device (CNC) and a laser oscillator. An electric power measuring instrument (26) measures the electric power (Pi) supplied to the laser oscillators (4, 5, 6). Further, a power sensor (24) measures the laser output (Pa). The laser output efficiency is calculated from the supplied electric power (Pi) and the laser output (Pa). When the laser output efficiency becomes smaller than a predetermined tolerable range of change, an instruction is displayed on the display screen to clean the optical parts. When the laser output efficiency becomes smaller than the tolerable range of change, it is so determined that the optical parts are subject to critical contamination and this fact is given to the operator through the display device.