PURPOSE: To form an orthogonal polarization type optical frequency shifter without using any half-wavelength plate by increasing the electric power of a high frequency signal to a transducer of a two-channel acousto-optic element by the level of a control signal when the light intensity of, for example, P polarized light becomes larger than that of S polarized light.

CONSTITUTION: For example, when the light intensity of a 0-order light beam 35 (P polarized light) becomes larger than that of a 0-order light beam 37 (S polarized light), the electric power of the high frequency signal to the transducer 22 of the two-channel acousto-optic element 21 is increased by the level of the control signal and the light intensity of the 0-order light beam 35 (P polarized light) is decreased [to increase the light intensity of a 1-order diffracted beam 36 (P polarized light) as a result]; and the 0-order light beam 35 (P polarized light) and 0-order diffracted light beam 37 (S polarized light) are equalized in light intensity to each other [to equalize the 1-order diffracted light beams 36 (P polarized light) ad 38 (S polarized light) in light intensity to each other]. Consequently, a transmitted beam 39 projected from the orthogonal polarization type frequency shifter have its components, i.e. P polarized light (light frequency: f1+80MHz) and S polarized light (light frequency: f1+81MHz) controlled equally at any time as to light intensity.