PURPOSE: To prevent a leakage of a resin from a resin path and an intrusion of air into the resin path by a method wherein semicylindrical valve discs rotatably fitted into semicylindrical fitting grooves each having a center shaft on a parting surface of a stationary mold with an intermediate mold and facing a resin path, and a resin path is formed in one of the valve discs.

CONSTITUTION: A stationary-side fitting groove 72 and a movable-side fitting groove 73 each of which is semicylindrical are respectively provided in a stationary mold 11 and an intermediate mold 12. A stationary-side valve disc 74 and a movable-side valve disc 75 each of which is semicylindrical are rotatably and slidably fitted into the stationary-side fitting groove 72 and the movable-side fitting groove 73, respectively. A resin path 83 for connecting a resin path 63 of the stationary mold 11 to a resin path 64 of the intermediate mold 12 at the time of mold clamping is formed in the movable-side valve disc 75. Prior to mold opening, both the valve discs 74, 75 are rotated through 90°. The resin path 64 of the intermediate mold 12 is closed by the movable-side valve disc 75. The resin path 63 of the stationary mold 11 is closed by the stationary-side valve disc 74. The resin path 83 of the movable-side valve disc 75 is also closed at the both ends thereof by the peripheral surface of the movable- side fitting groove 73.