PURPOSE: To lessen a pressure loss and to enable execution of injection molding under a low pressure by a method wherein a prescribed quantity of molten resin is stored and the stored molten resin is injected into and made to flow in a molding part by using the pressure of the fluid and molded therein.

CONSTITUTION: A resin passage of a nozzle 25 is closed by a shut-off valve 26, while a resin passage 19 is closed by a three-way valve 20, and a breathing mechanism 27 communicates with a resin inlet 15 through a selector device 22, a gas passage 23 and the three-way valve 20 and reduces a pressure in a storage chamber 14. When the resin passage 19 is opened by the three-way valve 20, molten resin 28 flows into the storage chamber 14 by a prescribed quantity with the rotating motion of a screw 12. Then, the resin passage 19 is closed by the three-way valve 20, while a nitrogen gas supply source 21 is connected with the resin inlet 15 through the selector device 22 and the gas passage 23 and a nitrogen gas of high pressure is introduced into the storage chamber 14. By opening the shut-off valve 26 of the nozzle 25, the molten resin 28 in the storage chamber 14 is injected under the pressure of the nitrogen gas into a cavity 8 from a resin outlet 16 through the nozzle 15 and filled up therein.