PURPOSE: To remove contamination strongly adhering to the surface of x-ray exposure mask by reinforcing the X-ray transmitting thin film with a supporting plate from the rear surface and then releasing pressured water to the surface of the X-ray transmitting thin film having the X-ray absorbing pattern.

CONSTITUTION: The periphery of supporting rod 3 made of Si wafer of the X-ray exposure mask is held by the X-ray exposure mask fixing holder 6 and the surface region corresponding to the region within the window at the rear side of X-ray transmitting thin film 2 is supported by a supporting plate 9 which is made by layered acrylic plate 7 in the thickness of 5mm and a teflon layer 8 in the thickness of 0.5mm. Under this condition, the surface of thin film 2 having the X-ray absorbing pattern 1 is exposed to water 10 pressurized to 150 PSi for one minute to wash the surface of X-ray exposure mask.