To provide a wavelength plate such that the angle of the polarization plane of the output light is always constant even if the angle of the polarization plane of the incident light deviates for a 1/2 wavelength plate formed by sticking two wavelength plates together.

The 1/2 wavelength plate is constituted by sticking together two wavelength plates, i.e. a 1st wavelength plate having a phase difference τ1 (deg) and an optical axis azimuth angle θ1 (deg) and a 2nd wavelength plate having a phase difference τ2 (deg) and an optical axis azimuth angle θ2 (deg). Here, the 1st wavelength plate and 2nd wavelength plate are stuck together so that the angle θ (deg) between the polarization planes of the incident light and output light is 2(θ2-θ1)-180 while τ1=360m+180 (m=0, 1, 2, ...) and τ2=360n+180 (n=0, 1, 2, ...).