To provide a maleimide-based copolymer resin film having high physical characteristics and high optical characteristics compatible with each other, to provide an optical film, to provide a polarizer protecting film, to provide a retardation film, and to provide a polarizing plate.

This maleimide-based copolymer resin film is formed by dispersing a rubbery polymer in a matrix formed out of a maleimide-based copolymer resin in a state of having a dispersion diameter of ≤100 μm, wherein the film has a parallel light transmittance of ≥ 87%.