Abstract: An apparatus comprising a resin plasticizing injection machine having an injection nozzle, a branch portion connected to the injection nozzle and having a plurality of outlet ports, molds and mold clamping devices both arranged around the branch portion in corresponding relation to the outlet ports, and an extension nozzle for maintaining each of the outlet ports in communication with each of the molds. Pressure control means is provided at an intermediate portion of the extension nozzle for maintaining the mold cavity at a specified pressure after resin has been injected into the mold. Means is provided for closing a flow channel extending from the injection machine to the pressure control means. After resin has been injected into a mold, the cavity of the mold can be maintained at the specified pressure by the pressure control means in place of the injection machine, so that the injection machine and the next mold can be subsequently brought into operation.