An injection molding machine includes a plasticating unit for plasticating a thermoplastic resin, and an injecting unit connected to the plasticating unit through a connecting passage to inject the plasticated resin into a mold. In the connecting passage, provided is a buffering unit for reserving the resin in an amount at least equal to the quantity of the resin injected per one shot. The plasticated resin is temporarily reserved in the buffering unit, and then, the reserved resin is fed to the injecting unit when the measuring is carried out, whereby the plasticating unit can be driven continuously simultaneously with the operation of the injecting unit and the mold. The molding cycle can shortened with the plastication efficiency being enhanced.