A polymer forming apparatus comprising: a chamber for storing a molten polymer resin; a foaming agent injector connected to the chamber and configured to receive a foaming agent from outside and supply the received foaming agent into the chamber when a supplying pressure of the foaming agent is higher than or equal to a pressure inside the chamber, the foaming agent injector comprising: a foaming agent injection pipe having an inlet channel forming an inlet for the foaming agent, an outlet channel forming an outlet for the foaming agent, and a middle channel connecting the inlet channel to the outlet channel, the middle channel having a smaller inner diameter than the inlet channel and the outlet channel; and an elastic shut-off unit elastically disposed inside the outlet channel at an outlet side of the middle channel, the elastic shut-off unit configured to open or close the middle channel according to the supplying pressure of the foaming agent; and a polymer backflow prevention unit disposed at the end of the outlet channel and configured to allow the supplied foaming agent to pass therethrough while preventing the polymer resin from flowing back by increasing a contact surface area with the polymer resin, the polymer backflow prevention unit comprising a backflow-prevention plate body having a plurality of backflow prevention holes formed therein, characterised in that the polymer backflow prevention unit supports the elastic shut-off unit. a chamber for storing a molten polymer resin; a foaming agent injector connected to the chamber and configured to receive a foaming agent from outside and supply the received foaming agent into the chamber when a supplying pressure of the foaming agent is higher than or equal to a pressure inside the chamber, the foaming agent injector comprising: a foaming agent injection pipe having an inlet channel forming an inlet for the foaming agent, an outlet channel forming an outlet for the foaming agent, and a middle channel connecting the inlet channel to the outlet channel, the middle channel having a smaller inner diameter than the inlet channel and the outlet channel; and an elastic shut-off unit elastically disposed inside the outlet channel at an outlet side of the middle channel, the elastic shut-off unit configured to open or close the middle channel according to the supplying pressure of the foaming agent; and a foaming agent injection pipe having an inlet channel forming an inlet for the foaming agent, an outlet channel forming an outlet for the foaming agent, and a middle channel connecting the inlet channel to the outlet channel, the middle channel having a smaller inner diameter than the inlet channel and the outlet channel; and an elastic shut-off unit elastically disposed inside the outlet channel at an outlet side of the middle channel, the elastic shut-off unit configured to open or close the middle channel according to the supplying pressure of the foaming agent; and a polymer backflow prevention unit disposed at the end of the outlet channel and configured to allow the supplied foaming agent to pass therethrough while preventing the polymer resin from flowing back by increasing a contact surface area with the polymer resin, the polymer backflow prevention unit comprising a backflow-prevention plate body having a plurality of backflow prevention holes formed therein, characterised in that the polymer backflow prevention unit supports the elastic shut-off unit.