To create higher pressure liquid beams in a laser beam machining device which machines a stock to be machined by superposing laser beams on the liquid beams and irradiating them on the stock to be machined.

The device is provided with a laser beam emitting means 2 which emits the laser beams L, a liquid beam emitting means 11 which emits the liquid beams B, and a reflecting mirror 8 which reflects the laser beams L emitted from the laser beam emitting means 2. The reflecting mirror 8 is provided with a passage 12 through which the liquid beams B emitted from the liquid beam emitting means 11 pass, the outlet opening 12a of the passage 12 is formed in the reflecting surface 8a of the reflecting mirror 8, the liquid beams B are made to pass through the passage 12 and are made to go out of the outlet opening 12a formed in the reflecting surface 8a, and also the laser beams L are reflected toward the advancing direction of the liquid beams B around the outlet opening 12a of the reflecting surface 8a and are converged on the liquid beams B.