The present invention provides an apparatus for forming a polymer, comprising: a chamber having a predetermined pressure and storing a molten polymer resin; a foaming agent injection portion connected to the chamber and supplying a foaming agent, which is supplied from the outside, into the chamber by forming a pressure equal to or higher than the pressure inside the chamber; and a polymer backflow prevention portion provided at the foaming agent injection portion, and through which the supplied foaming agent passes so as to increase a contact surface area with the polymer resin, thereby preventing the backflow of the polymer resin.