The method of the present disclosure is a method for producing a porous polymer film including: irradiating a strip-shaped polymer film with an ion beam while moving the polymer film transversely to the ion beam, so as to form a polymer film that has collided with ions in the beam; and chemically etching the formed polymer film so as to form openings and/or through holes corresponding to tracks left by the colliding ions in the polymer film. The ion beam (11) with which the polymer film is irradiated is obtained by folding a tail of an original beam (51) inwardly toward a center of the original beam by nonlinear focusing. The original beam is composed of ions accelerated in a cyclotron and has a cross-sectional intensity distribution profile in which an intensity is maximum at the center of the original beam and continuously decreases from the center toward the tail of the original beam, and the profile is an intensity distribution profile in a cross section perpendicular to a direction of the original beam.