A method of manufacturing a biaxially stretched film according by: (I) preheating a thermoplastic resin film to a temperature in a rubbery plateau region in a storage elastic modulus curve; (II) biaxially stretching the thermoplastic resin film under a condition that a stretching speed is 500%/min or higher while heating the thermoplastic resin film to the temperature in the rubbery plateau region; (III) cooling the thermoplastic resin film after the biaxially stretching (II) to a temperature in a glass-to-rubber transition region or a glass region in the storage elastic modulus curve; and (IV) relaxing the thermoplastic resin film after the cooling (III) at a temperature in the glass-to-rubber transition region.

The present invention provides a method of manufacturing a biaxially stretched film which exhibits small heat shrinkage and has excellent dimensional stability. An unstretched thermoplastic resin film composed of at least one thermoplastic resin layer is biaxially stretched in a longitudinal direction and a width direction to manufacture the biaxially stretched film. The method of manufacturing the biaxially stretched film according to the present invention includes: a process (I) for preheating the thermoplastic resin film to a temperature in a rubbery plateau region in a storage elastic modulus curve; a process (II) for biaxially stretching the thermoplastic resin film under a condition that a stretching speed is 500%/min or higher while heating the thermoplastic resin film to the temperature in the rubbery plateau region; a process (III) for cooling the thermoplastic resin film after the process (II) to a temperature in a glass-to-rubber transition region or a glass region in the storage elastic modulus curve; and a process (IV) for relaxing the thermoplastic resin film after the process (III) under the temperature in the glass-to-rubber transition region.