To significantly heighten the performance of electron transmissivity by reducing the thickness of a window foil through a less expensive and easy working without causing defects such as crinkles and pin holes to the window foil of an electron beam irradiation window and facilitate the handling of the window foil and prevent it from being damaged even if its thickness is reduced.

By irradiating the window foil 2 with an energy beam like an ion beam from the other side of a support plate 3 with the window foil 2 papered over one side of the support plate 3 for supporting it or pressed against one side of a substitute jig for the support plate 3, each electron beam irradiation hole 4 bored through the support plate 3 of the window foil 2 and parts 8 facing the hole 4 are milled to make them thinner than other parts.