A phase difference plate is provided which can be formed by a single material at a low cost without a need for forming laminated layers, and which has excellent performance in a wide band. The phase difference plate is formed by using a norbornene based resin composition which includes a norbornene based resin; and a polymer whose birefringence value is negative, and whose wavelength dispersion of birefringence values satisfies |&Dgr;n(450)/&Dgr;n(550)|≧1.02, wherein &Dgr;n(450) and &Dgr;n(550) are birefringence values (&Dgr;n) at a wavelength of 450 nm and a wavelength of 550 nm, respectively. The polymer is preferably a polystyrene based polymer.