A center-portion molding member having a center-portion transfer shape surface and a side wall surface is inserted and fitted in a frame-portion molding member having a frame-portion transfer shape surface to form a part of a cavity. The center-portion molding member has a groove portion in the side wall surface. The groove portion protrudes from the frame-portion transfer shape surface. Before injected resin transfers a shape of the center-portion transfer shape surface, the side wall surface of the center-portion molding member is sealed with the resin to form, between the center-portion transfer shape surface and the resin, a space in which gas is confined.