A laser machining device includes a contamination determining part that determines contamination of an external optical system before laser machining; the contamination determining part includes a lens contamination determining section that determines contamination of a lens in the external optical system on the basis of a comparison between a first measurement value, which is measured by an energy amount measuring part in a state where the external optical system is not heated, and a second measurement value, which is measured by the energy amount measuring part in a state where the external optical system is heated.