An optical film is provided, which displays a positive wavelength dependency across the entire wavelength region from 400 to 800 nm, and is capable of imparting a specified retardation to transmitted light with a single sheet of film. The optical film particularly, includes a thermoplastic norbornene-based resin with a specified structure formed of a structural unit (I) which imparts a positive birefringence and a structural unit (II) which imparts a negative birefringence, and which satisfies particular conditions with respect to .DELTA.N.sub.I(.lamda.), .DELTA.N.sub.II(.lamda.), .DELTA.N.sub.I(800) and .DELTA.N.sub.II(800) wherein .DELTA.N.sub.I(.lamda.) and .DELTA.N.sub.II(.lamda.) represent the difference between a refractive index Nx(.lamda.) in an x axis direction at a wavelength .lamda., and a refractive index Ny(.lamda.) in a y axis direction, and .DELTA.N.sub.I(800) and .DELTA.N.sub.II(800) represent the difference in refractive indexes at a wavelength of 800 nm, and the x axis represents the stretching direction and the y axis represents the in-plane direction perpendicular to the x direction.