A method for producing a thermoplastic polymer material is provided, the method including the steps of supplying component (A) below to a vent type extruder that includes a starting material supply opening, a liquid inlet, and a vent via the starting material supply opening of the extruder, supplying component (B) below to the extruder via the liquid inlet of the extruder, melting and kneading component (A) and component (B) within the extruder, and degassing a volatile component via the vent and extruding through a die a thermoplastic polymer material comprising a thermoplastic polymer of component (A) and at least one compound selected from compound group S of component (B). (A) A polymer composition that includes a thermoplastic polymer and an organic solvent and/or a monomer. (B) A solution of at least one compound selected from compound group S consisting of trehalose, sucrose, lactose, maltose, etc.