PURPOSE: To prevent resin leaking and air entraining by installing a valve, which opens a joint connected to a resin passage on the nozzle side, in a resin passage on the main manifold side and by installing the valve body, which closes the joint by contacting from the main manifold side, and a spring, which energizes the valve body toward the resin passage on the nozzle side, on the valve seat.

CONSTITUTION: A fixed-side extension manifold 41, to which the nozzle of a molding machine is always connected, is attached on a fixed mold 1, and a movable-side extension manifold 61 is fixed to the main manifold 28 of a middle mold 2. The extension manifolds 41, 61 are attached or detached corresponding to mold locking or opening. A valve 85 installed on the top of the movable-side extension manifold 61 consists of a ball type valve body 84, which contacts a taper-shaped valve seat 77, and a spring which presses the valve body 84. The valve body 84 is opened by resin pressure during an injection process and a dwelling process and is closed during a mold opening process. In this way, the joint is closed strictly while the resin pressure is not applied, so that the resin can be prevented from leaking out of the resin passage on the side of the main manifold during the mold opening.