A prism pair is employed as an optical deflecting element. The prism pair is constructed by a slab waveguide, and first and second upper electrodes and first and second lower electrodes arranged on and under the slab waveguide. These electrodes are shaped into a wedge shape (e.g., triangular shape), and change the refractive index of a part of the slab waveguide by utilizing the electrooptic effect to change the propagation direction of light.