Method for producing a phase difference film

A method for producing a phase difference film,

• the phase difference film consisting of a resin C contains a copolymer P containing a polymerization unit A and a polymerization unit B,
• the phase difference film including a phase separation structure that exhibits a structural birefringence, the phase separation structure including a phase including as a main component the polymerization unit A and another phase including as a main component the polymerization unit B,
• the phase difference film having an NZ factor of greater than 0 and smaller than 1, and
• the method comprising:
• a step of forming a single layer film of a resin C;
• and
• a step of causing phase separation of the resin C in the film,
• wherein the step of causing phase separation of the resin C includes a step of applying to the film a stress along a thickness direction thereof.