A wave plate comprising: a prism member having an entrance surface for receiving a terahertz wave T, and an exit surface for emitting the terahertz wave T received by the entrance surface; wherein the prism member is constituted by a plurality of waveguide regions having: a partial entrance surface for receiving a part of the terahertz wave T, a plurality of total reflection surfaces for totally reflecting the terahertz wave T from the partial entrance surface, and a partial exit surface for emitting the terahertz wave T totally reflected from the total reflection surfaces; and each of the partial entrance surfaces combine to constitute the entrance surface of the prism member, and each of the partial exit surfaces combine to constitute the exit surface of the prism member, by stacking waveguide regions.