A method of manufacturing a vehicle seat member including a foamed resin molded body and a frame member at least a part of which is embedded in the foamed resin molded body, wherein the frame member includes a framework (250, 260) extending along an outer shape of the vehicle seat member in plan view, the method of manufacturing a vehicle seat member comprising: a placing step of placing the frame member in a molding die so that the framework (250, 260) extends along the outer shape of the vehicle seat member in plan view; a molding step of molding the foamed resin molded body in the molding die so that at least a part of the frame member is embedded; and a gap forming step of forming a gap (108, 109, 110, 111) between at least a part of the framework (250, 260) and a part (112, 113) of the foamed resin molded body located closer to an outer side and/or an inner side of the vehicle seat member in plan view than the at least a part of the framework (250, 260), the gap (108, 109, 110, 111) extending along the extending direction of the framework (250, 260), the gap forming step being performed in the molding step or after the molding step, wherein the outer shape of the vehicle seat member has a rectangular shape; the framework (250, 260) of the frame member includes two or more framework corner parts (205, 206, 207, 208) connected to each other and extending along each of two or more corners selected from four corners (405, 406, 407, 408) of the outer shape, wherein the two or more corners are selected so as to constitute one or more pairs of corners opposed in a direction of any one side of the outer shape; characterized in that the gap forming step includes formation of the gap (108, 109, 110, 111) along the extending direction of each of the framework corner parts (205, 206, 207, 208) between each of the framework corner parts (205, 206, 207, 208) and a part (112, 113) of the foamed resin molded body located closer to an outer side and/or an inner side of the vehicle seat member in plan view than each of the framework corner parts (205, 206, 207, 208). a placing step of placing the frame member in a molding die so that the framework (250, 260) extends along the outer shape of the vehicle seat member in plan view; a molding step of molding the foamed resin molded body in the molding die so that at least a part of the frame member is embedded; and a gap forming step of forming a gap (108, 109, 110, 111) between at least a part of the framework (250, 260) and a part (112, 113) of the foamed resin molded body located closer to an outer side and/or an inner side of the vehicle seat member in plan view than the at least a part of the framework (250, 260), the gap (108, 109, 110, 111) extending along the extending direction of the framework (250, 260), the gap forming step being performed in the molding step or after the molding step, wherein the outer shape of the vehicle seat member has a rectangular shape; the framework (250, 260) of the frame member includes two or more framework corner parts (205, 206, 207, 208) connected to each other and extending along each of two or more corners selected from four corners (405, 406, 407, 408) of the outer shape, wherein the two or more corners are selected so as to constitute one or more pairs of corners opposed in a direction of any one side of the outer shape; characterized in that the gap forming step includes formation of the gap (108, 109, 110, 111) along the extending direction of each of the framework corner parts (205, 206, 207, 208) between each of the framework corner parts (205, 206, 207, 208) and a part (112, 113) of the foamed resin molded body located closer to an outer side and/or an inner side of the vehicle seat member in plan view than each of the framework corner parts (205, 206, 207, 208).