Provided are a method for producing a polymer material having a high degree of crystallization, a small variability in degree of crystallization, and a three-dimensionally isotropic crystallinity to thus give high thermal resistance, high isotropy of resin physical properties, and a small variability in resin physical properties; and the polymer material. The method for producing a polymer material includes the steps of: heating a thermoplastic resin to or above a melting point determined from a melting peak measured by DSC into a thermoplastic resin melt; applying a pressure higher than atmospheric pressure to the thermoplastic resin melt; starting cooling the thermoplastic resin melt with a pressure higher than atmospheric pressure applied thereto; releasing the pressure on the thermoplastic resin melt after cooling the thermoplastic resin melt to a cooling-end-temperature lower than an upper limit of a crystallization temperature range, wherein during the cooling, the pressure on the thermoplastic resin melt is maintained at the pressure higher than atmospheric pressure or raised. The polymer material is one obtained by the above production method.