A method is provided for manufacturing a wide band retardation plate which gives uniform phase difference characteristics to incident light over the whole visible wavelength region, and which, as it permits selection of raw materials regardless of whether they have a positive or negative intrinsic double refraction value, allows a wide selection of raw materials. For this purpose, the method comprises a machine direction-stretched film-forming step for transporting and stretching in an identical direction to the transport direction, a Material A of two or more materials having different positive intrinsic double refraction values to form a machine direction-stretched film, a transverse direction-stretched film-forming step for transporting and stretching in a perpendicular direction to the transport direction, a Material B of the aforesaid two or more materials to form a transverse direction-stretched film, and a lamination step for laminating the machine direction-stretched film and the transverse direction-stretched film.