PURPOSE: To decrease the damages to be given on the surface of wiring patterns and to effectively remove foreign matter by passing the phase shift mask under water flow having ultrasonic vibrations in horizontal, perpendicular or other positions.

CONSTITUTION: The water flow 1 having the ultrasonic vibrations is formed by using, for example, 'High Megasonic US Shower (R)' produced by Kaijo Co., and applying 95kHz ultrasonic vibrations to the water. The phase shift mask 2 arranged parallel on a transporting machine 3 is passed through this water flow. The water molecules having the ultrasonic vibrations vibrate the foreign matter intruding into the recessed parts of the mask. The foreign matter is thereby separated and removed from the mask together with the washing water flow. The resticking of the foreign matter once removed from the substrate is prevented by the water flow and the sticking of the foreign matter is decreased as compared with the conventional methods. As a result, the foreign matter on the surface is effectively removed without giving serious damages on the wiring patterns of the phase shift mask 2.