PURPOSE: To restrain a reduction in a Cs thin film and prolong the service life of a mask by supplying Cs gas to an atmosphere in contact with a photoelectric surface composed of the Cs thin film.

CONSTITUTION: When a photoelectric transfer mask whose photoelectric surface is composed of a Cs thin film is irradiated with light to emit photoelectrons from the photoelectric surface and a pattern formed on the mask is transferred to a wafer by using the photoelectrons, cesium gas is supplied to an atmosphere in contact with the photoelectric surface. For example, a Cs gas beam generator 21 is arranged in an exposure chamber 1 and the Cs gas is emitted toward the photoelectric surface of the mask 2 to increase the partial pressure of the Cs gas near the photoelectric surface. The emission of the Cs gas in continued during an exposure. The Cs gas beam generator 21 generates the Cs gas, for example, by the evaporation of Cs by heating or the reduction of cesium chromate and converts the Cs gas to a beam by the guidance of a cylindrical body.