PURPOSE: To realize maximum strength ratio while keeping constant the voltage applied to the front part and rear part of the coupling part, by shifting the dividing ratio of the coupling part, or the dividing ratio of the electrode, by the extent corresponding to the intitial phase difference between waveguides.

CONSTITUTION: The coupling part is divided into two parts, of which lengths should be respectively l1 and l2. Their relation should be δl=l1 - l2. The initial phase difference of propagation constant of two waveguides should be δβ0 = β1 - β2. Electrodes 4-2, 4-3 are commonly grounded, and the same voltage is applied to electrodes 4-1, 4-4 from outside. Phase differences equal in size and differing in sign, Δβ(E) and -Δβ(E), are applied between the waveguides, before and after the coupling part. At this time is found the presence of dividing deviation δl which maximizes the strength ratio when δβ0 ≠ 0. Therefore, after determining the initial phase difference δβ0 by preliminary experiment, when the electrodes are made by deducing the dividing deviation δl corresponding to side phase difference, the maximum strengh ratio may be realized while keeping constant the voltage applied to the electrodes 4-1, 4-2.