The present invention is intended to provide a thermoplastic resin composition which exhibits excellent compatibility while retaining low birefringence and is excellent in weathering resistance and heat resistance, an optical film which is obtained from the thermoplastic resin composition and a retardation film which is excellent in weathering resistance and heat resistance and has reciprocal wavelength dispersion properties. The thermoplastic resin composition comprises a vinyl-based polymer (A) having a unit represented by the following formula (I) and a cycloolefin-based polymer (B). wherein R1 to R3 are each a hydrogen atom, a halogen atom, a substituted or unsubstituted hydrocarbon group of 1 to 30 carbon atoms which may have a linkage containing an oxygen atom, a sulfur atom, a nitrogen atom or a silicon atom, or a polar group, and n is 0 or a positive integer.