To reduce the strain of a mask by providing a mask holding part composed of an outer frame for holding the mask contacted with the marginal edge of one surface of the frame and inner frame for holding the mask contacted with the inside of one surface: the inner frame being connected to the outer frame.

A mask 1 has a beam 1a and thin wall part 1b. An electron beam is emitted on the latter part 1b for transferring a desired pattern formed on this part to a semiconductor wafer. A mask-holding part 2 is composed of outer and inner frames 2b, 2c and covers at least a mask holding face 2a with silicon carbide or diamond. The mask 1 is aligned, so as to agree the beam 1a with the inner frame 2c of the holding part 2 to hold the frames 2b, 2c. The frames 2b, 2c are covered with a conductive film e.g. Al film which is grounded.