To provide a method for using a wavelength plate composed of a polymer resin film, which is easily worked compared with a quartz optical wavelength plate, is flexible and easy to handle, is sufficiently capable of coping with thin film forming and still does not loose characteristics of the wavelength plate according to an angle of incident light.

In the method for using the wavelength plate, which comprises inserting the wavelength plate composed of the polymer resin film in an optical path and varying a polarization state of light passing through the wavelength plate, the wavelength plate is used so as to make an angle θ between the direction of the light incident on the wavelength plate and the direction vertical to the resin film be 0°<θ<90°. The wavelength plate is preferably used by keeping the angle θ with respect to the direction vertical to the resin film be 0°<θ<10°. The polymer resin film is preferably a polyimide film or a polyamide- imide silicone polymer film which is a film preferably uniaxially stretched or uniaxially stretched under heating.