Provided are a retardation film that has a
high heat resistance, has excellent
formability and handleability even in a single-layer structure, has a negative thickness-direction retardation Rth value, and is suitable as a negative A-plate or a positive C-plate and a method for producing the film. The retardation film is formed of a stretched film of a
polyester resin, contains a unit (A1) represented by the formula (1) as a
diol unit (A) and a unit (B1) represented by the formula (2a) or (2b) as a
dicarboxylic acid unit (B), and is a negative A-plate or a positive C-plate.In the formulae, Z1 and Z2 represent an
aromatic hydrocarbon ring, R1, R2a, R2b, R3a and R3b represent a
substituent, k, p1 and p2 denotes an integer of 0 to 8, q denotes an integer of 0 to 4, m1, m2, n1 and n2 denotes an integer of not less than 0, A1a and A1b represents an alkylene group, and A2a, A2b and A3 represents a
divalent hydrocarbon group.