To provide a retardation film that, while maintaining a high retardation value, has increased durability such as moist heat resistance and can prevent the occurrence of color unevenness even if made in a size having a large area. The retardation film contains a cellulose ester as the primary component, has a film thickness of 15 μm or more and less than 40 μm, has a film thickness variation both in the widthwise direction and lengthwise direction of 0 to 4 μm, and satisfies an Rt humidity fluctuation represented by the formula (1) described below of 1% to 12%. Formula (1): Rt humidity fluctuation=(&Dgr;Rt value, a difference of retardation Rt(590) values in the thickness direction represented by the following formula (ii) measured at a wavelength of 590 nm after the film has been left to stand for 5 hours in each of a 23° C., 20% relative humidity environment and a 23° C., 80% relative humidity environment)/(Rt(590) value measured after the film has been left to stand for 5 hours in a 23° C., 55%
