A method is provided for producing easily a membrane (film) having a micro surface structure (porous structure, fibrous structure and the like) in nano-order. The method for producing a film having a nano-structure on the surface of the film, includes the steps of: (1) coating a substrate with a solution containing a copolymer including two or more homopolymer segments and an organic solvent having boiling point of 82° C. or more and a dielectric constant of 30 or less to form a membrane; (2) providing the membrane with a water vapor-containing gas having a relative humidity of 50% or more to age the membrane; and (3) drying the membrane to obtain the film.