PURPOSE: To obtain a molded piece short of a navel part by a method wherein a fixed quantity of molding material is supplied into a mold through an injection hole, the injection hole is closed by a plug in a state of compression and the molding material can be kept from flowing backward to the injection hole.

CONSTITUTION: Molding material (synthetic material or rubber) (a) is supplied from an extruder 8 into molds 1, 2 through an injection hole 3. The nozzle 9 of the extruder 8 is retreated and the plug body 11 of a stopper 10 closes the injection hole 3 in a state of compression. The mold 2 is moved upward and the molds 1, 2 are closed tightly to compress and mold the molding material (a). In this case, cooling water is passed through water holes 6, 17, cooled and hardened or the mold is heated and hardened.

EFFECT: Since the molded piece has no navel part, a molded piece of fine finish, free from stress concentration to the root of the navel and of uniform strength can be obtained. This piece is manufactured efficiently because the navel cutting process is not required.