The present invention provides a film for a back sheet of a solar cell exhibiting excellent water vapor barrier properties, water resistance and gas barrier properties, has flexibility and mechanical strength and has excellent heat stability, and a method for producing the film.The present invention is a water vapor barrier film comprising a clay membrane coated on a PET substrate, wherein the clay membrane is formed of clay and an additive; the weight ratio of the clay relative to the total sold is 60 to 90% by weight in the clay membrane; the production process thereof comprises a water resistance imparting heat treatment at 100 to 200° C. after the coating and drying; the film has a water vapor transmission rate of less than 3 g/m2·day; the additive in the modified clay is a polyimide; and at least 90% by mole of exchangeable ions is lithium ion, and a method for producing the water vapor barrier film.The above film exhibits water vapor barrier properties and is useful as a back sheet for a solar cell and the like.