PURPOSE: To produce a masking film which is hardly influenced by humidity change.

CONSTITUTION: On both surfaces of a plastic film 1, coating layers 2, 2' having lower permeation of water vapor than the plastic film are uniformly formed, and at least one coating layer is covered with a light-shielding permeates very little water vapor from air, and the amt. of vapor to the plastic film in the center of the film is limited. Thereby, stable dimension can be maintained for a longer time than a conventional film and is hardly influenced by humidity. Thus, no dimensional change or abnormality in flatness is caused. Thus, the plate making process which requires high dimensional accuracy can be performed in a normal working environment.