To obtain a laser beam machining device capable of stable machining over a long term by transmitting a laser beam outputted from a laser oscillator to a condensing lens without distortion and avoiding a local temperature rise in the optical path duct through a high cleaning effect.

In a laser beam machining device provided with a duct 3 in the optical path (optical duct) through which a laser beam 2 guided from a laser oscillator 1 reaches a workpiece 8, a temperature instrument 12 is provided which measures temperature at plural places along the longitudinal direction inside the optical duct 3, as is a temperature control device 13 which maintains at a uniform and fixed value the temperature distribution of a gas inside the optical duct 3 on the basis of the output of the temperature instrument 12.