PURPOSE: To reduce a physical property difference in a widthwise direction by decreasing a bowing phenomenon, by a method wherein a cooling process having a length L and satisfying a specific formula is provided between a lateral stretching process and thermal fixation process which manufacture a thermoplastic resin film stretched at least in a lateral direction and the film is cooled at the glass transition temperature or less.

CONSTITUTION: At the time of lateral stretching and thermal fixation treatment of a thermoplastic resin film, simultaneously with cooling of the film prior to thermal fixation to the glass transition temperature or less once a nip roll group is installed and a bowing phenomenon to be generated with a lateral stretching process is reduced. The lower a cooling temperature becomes, the more a reduction effect of the bowing phenomenon is improved. A length L of a cooling process is selected so that the bigger a value of a ratio L/W between the length L of the cooling process and a width W of the film becomes and wider the maximum width WN of a nip within the nip roll group becomes further, the more the effect is improved and a formula (L/ W)≥0.25(2-WN/W)2 is obtained. It is preferable that a ratio WN/W of the width of the nip to the width of the film is at least 0.2.