A phase retarder comprises one cellulose ester film. The retardation value of the film represented by Re450 measured at the wavelength of 450 nm is in the range of 100 to 125 nm. The retardation value of the film represented by Re590 measured at the wavelength of 590 nm is in the range of 120 to 160 nm. The retardation values represented by Re450 and Re590 satisfy the condition of Re590-Re450.gtoreq.2 nm. An aromatic compound having at least two aromatic rings is incorporated in the film as a retardation increasing agent to obtain .lamda./4 in the wide wavelength region defined above. A phase retarder can also give .lamda./2 in a wide wavelength region in the same manner.