A laser beam machining apparatus includes a laser oscillator, a machining head which that machines a workpiece using the laser beam. An optical duct has an optical system to guide the laser beam from the laser oscillator to the machining head. Purge gas is supplied into the optical duct from a purge gas supply port, and the purge gas is output from a purge gas exhaust port. A detector detects presence of undesired gas in the optical duct.