Manufacturing method for flanged resin product and flanged resin product

A manufacturing method for a resin product (20) comprised of a plurality of separate pieces (21,22,23) and having a flange (22a), the plurality of separate pieces (21,22,23) including one as a separate piece (22) having a flange (22a) corresponding to the flange (22a) of the resin product (20), comprises primary-injection-molding the plurality of separate pieces (21,22,23) including the separate piece (22) having the flange (22a) by a primary injection of a resin, and having respective separate planes of the plurality of separate pieces in abutment, joining the separate pieces (21,22,23) together to mold the resin product (20) having the flange (22a) by a secondary injection of a welding resin. When molding the plurality of separate pieces (21,22,23) including the separate piece (22) having the flange (22a) by the primary injection of the resin, a gate part (22b) for molding the flange (22a) of the separate piece (22) having the flange (22a) is positioned inside the flange (22a).