Retardation film and method for producing the same

A retardation film comprising a stretched film of a polyester resin containing a diol unit (A) and a dicarboxylic acid unit (B), wherein

• the diol unit (A) contains a first diol unit (A1) having a fluorene skeleton represented by the following formula (1):
• wherein Z1 and Z2 independently represent an aromatic hydrocarbon ring, R1, R2a and R2b independently represent a substituent, k denotes an integer of 0 to 8, m1, m2, n1 and n2 independently denote an integer of not less than 0, and A1a and A1b independently represent a straight-chain or branched-chain alkylene group,
• the dicarboxylic acid unit (B) contains a dicarboxylic acid unit (B1) having a fluorene skeleton represented b the following formula 2a or (2b):
• wherein R3a and R3b independently represent a substituent, p1 and p2 independently denote an integer of 0 to 8, q denotes an integer of 0 to 4, and A2a, A2b and A3 independently represent a divalent hydrocarbon group which may have a substituent,
• a proportion of the dicarboxylic acid unit (B1) relative to the total dicarboxylic acid unit (B) is not less than 90% by mol, and a proportion of the total amount of the first diol unit (A1) and the dicarboxylic acid unit (B1) relative to the total constitutional units of the polyester resin is not less than 85% by mol,
• wherein a weight-average molecular weight Mw of the polyester resin is a range from 50000 to 1000000,
• wherein the retardation film is free from other thermoplastic resins, is a positive C-plate or a positive B-plate, and
• has an average thickness of 10 to 60 μm.