A cavity (2) is formed with a convex pattern (X) uniformly formed by electric casting on an inner peripheral face of a mold main body (6) of an undivided and seamless configuration having a circular through hole (7). Next, a core (12) is inserted to the cavity (2). Then, molten resin (3) is injected into the cavity (2), a resin molded product (44) having a concave pattern (Y) on the surface and the core (12) in the center is molded, and the resin molded product (44) is departed from the inner face (10) of the cavity (2) by reduction of outer diameter dimension (Æd) with cooling shrinkage of the molten resin (3). And, the resin molded product (44) is drawn out of the cavity (2).