Abstract: A nozzle to be connected between a conventional molding machine and a mold cavity for injecting molten resin of thermoplastics into a mold cavity where a gate at the front end of a cylinder of the nozzle opens and closes automatically according to the pressure of molten resin. A differential moving piston slidably disposed in the cylinder includes a front piston and a rear piston mounted on the front end and the rear end thereof respectively, a passage for molten resin axially drilled therethrough and a needle protruding forward from the front end of the front piston for mating with the gate according to the sliding movement thereof. A spring disposed in the cylinder pushes the differential moving piston forward to close the gate.