A laser machining device comprising: a machining table on which a workpiece is to be placed; a laser oscillator that is arranged to emit a laser beam; a light-guide optical system that is arranged to deflect the laser beam emitted from the laser oscillator; an extensible/contractible bellows that has a shape of a cylinder and surrounds an optical path of the laser beam after the light-guide optical system deflects the laser beam; a bend mirror that is capable of moving in an axial direction of the bellows while extending or contracting the bellows being coupled therewith, and is arranged to deflect the laser beam having passed through the bellows in a direction toward the machining table; a machining head that is arranged to irradiate the workpiece with the laser beam deflected by the bend mirror; characterized by further comprising an abnormality detector (43a, 43b) including a light emitting unit that is arranged to emit a light beam advancing in parallel with an axis of the bellows, and a light receiving unit that is arranged to measure the amount of received light of the light beam; and a control unit that is arranged to bring down the laser oscillator when the amount of received light of the light beam in the light receiving unit falls below a first threshold, the first threshold being set such that, when smoke reaches a space sandwiched between the light emitting unit and the light receiving unit, the amount of received light in the light receiving unit falls below the first threshold, the first threshold being larger than a second threshold which is set such that, when a sliding fault occurs, the amount of light received by the light receiving unit becomes smaller than the second threshold. a machining table on which a workpiece is to be placed; a laser oscillator that is arranged to emit a laser beam; a light-guide optical system that is arranged to deflect the laser beam emitted from the laser oscillator; an extensible/contractible bellows that has a shape of a cylinder and surrounds an optical path of the laser beam after the light-guide optical system deflects the laser beam; a bend mirror that is capable of moving in an axial direction of the bellows while extending or contracting the bellows being coupled therewith, and is arranged to deflect the laser beam having passed through the bellows in a direction toward the machining table; a machining head that is arranged to irradiate the workpiece with the laser beam deflected by the bend mirror; characterized by further comprising an abnormality detector (43a, 43b) including a light emitting unit that is arranged to emit a light beam advancing in parallel with an axis of the bellows, and a light receiving unit that is arranged to measure the amount of received light of the light beam; and a control unit that is arranged to bring down the laser oscillator when the amount of received light of the light beam in the light receiving unit falls below a first threshold, the first threshold being set such that, when smoke reaches a space sandwiched between the light emitting unit and the light receiving unit, the amount of received light in the light receiving unit falls below the first threshold, the first threshold being larger than a second threshold which is set such that, when a sliding fault occurs, the amount of light received by the light receiving unit becomes smaller than the second threshold.