A birefringent plate includes a transparent glass substrate, an antireflection coating formed on one of the surfaces of the glass substrate, an obliquely deposited film formed on the other one of the surfaces of the glass substrate by obliquely deposition of a dielectric material with respect to the normal of the glass substrate, and a protective film formed on the obliquely deposited film for holding the occluded water of the obliquely deposited film within the obliquely deposited film and reducing the light interference between the opposite surface of the glass substrate and the surface of the obliquely deposited film. The antireflection coating and the protective film inhibit the phase difference from fluctuating at different measurement positions when the phase difference is measured by using a light source, such as a laser beam. In addition, the protective film makes the birefringent plate less dependent on the temperature.