PURPOSE: To obtain a circular or circular arc resin-molded product having a high precision of radius of curvature, by making perforations at given intervals in a metal base plate along the circular or the circular arc rib and by outsert-molding by the use of the aforesaid work.

CONSTITUTION: When providing a circular or a circular arc rib 6 on a metal base plate 2, perforations 1 made in the metal plate 2 shall be psotioned so that an angle, which is determined with a center 7 of the circle or circular arc of the rib as the starting point, must be 30 degrees or less. After the metal plate 2 is then fastened at the inside of the mold, resin is flown into through the perforations 1, and collars, which have a cross section being larger than an area of the perforations 1, are provided at both surfaces of the metal plate, and resin-made rib is molded in a precise size.