Abstract: A laser beam machining device employs a laser for generating a laser beam, a chopper device for intermittently blocking the laser beam for intermittently irradiating the laser beam onto the surface of a work, and an assisting gas discharge device for injecting assisting gas toward the point, on which the laser beam is irradiated. By locating the chopper upstream of the primary condenser and away from the assist gas flow, the assist gas to be discharged toward the predetermined point of the work, on which the laser beam is irradiated, is not disturbed which otherwise would cause turbulence to degrade the precision of the laser beam machining operation. In addition, the assisting gas discharging device is so arranged as to direct the assist gas flow substantially along the axis of the laser beam to be irradiated onto the work surface.