To eliminate a residue of a resin inner pressure or an inner strain and to perform a production cost equivalent to that of a thin molding and a high accuracy by incompletely transferring a cavity shape of a mold in a recess or protrusion shape to a site in which at least one or more resin inner pressures or inner strains easily occur.

After a molten resin is injection filled in a cavity, when cooling and solidifying are advanced so that a resin pressure becomes a given value, a movable insert is retracted from the resin in the cavity in a separate direction, and an air gap is provided between a sidewall face of the insert and the resin. A corresponding resin is deformed caused by the pressure upon separating of the insert side wall face. If the pressure is retained upon releasing of a local pressure, the resin becomes a protrusion shape, while if the pressure is an atmospheric pressure or less, the resin becomes a recess shape caused by a local mold releasing contraction to become a plastic lens 10 formed with an incompletely transferred part 21 and to have mirror faces 11, 12 of required shape accuracy. Thus, shape and optical accuracies can be retained without residual pressure or strain, and it can be made with a production cost equivalent to that of the thin molding.