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. ##STR00001## In the formulae, Z.sup.1 and Z.sup.2 represent an aromatic hydrocarbon ring, R.sup.1, R.sup.2a, R.sup.2b, R.sup.3a and R.sup.3b 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, A.sup.1a and A.sup.1b represents an alkylene group, and A.sup.2a, A.sup.2b and A.sup.3 represents a divalent hydrocarbon group.

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. ##STR00001## In the formulae, Z.sup.1 and Z.sup.2 represent an aromatic hydrocarbon ring, R.sup.1, R.sup.2a, R.sup.2b, R.sup.3a and R.sup.3b 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, A.sup.1a and A.sup.1b represents an alkylene group, and A.sup.2a, A.sup.2b and A.sup.3 represents a divalent hydrocarbon group.