There is provided by the invention a retardation film stably exhibiting excellent optical properties over a long period of time. The retardation film of the invention is a retardation film comprising (A) a cycloolefin resin and (B) inorganic particles which have a longer diameter and a shorter diameter and exhibit shape anisotropy, a refractive index of which in the longer diameter direction is larger than an average refractive index of which in the direction crossing the longer diameter direction at right angles and which exhibit birefringence, wherein the inorganic particles (B) are orientated and the retardation film has a difference in refractive index between the film plane direction and the film thickness direction.