PURPOSE: To obtain a phase difference film having large spectrum spread property of birefringence, excellent optical homogeneity and can be used for a liquid crystal device requiring quick responsiveness by specifying the reduced viscosity in N-methyl pyrolidone and forming from a polyarylene ether sulfone copolymer derived from 2,2'-dihydroxy diphenyl.

CONSTITUTION: This phase difference film has 0.1-2dl/g reduced viscosity measured in 1wt.% solution of N-methyl pyrolidone at 25°C and is composed of the polyarylene ether sulfone copolymer desired from 2.2'-dihydroxy biphenyl. The polyarylene ether-sulfone copolymer has very excellent solubility to a chlorinated hydrocarbon such as methylene chloride. One having 0.2-1.5dl/g reduced viscosity is particularly preferably used. When the molecular weight is smaller than the range, the strength of the film is lowered and the molecular weight larger than the range is unfavorable since the film forming property of the film is poor.