PURPOSE: To perform phase matching and coupling length adjustment and attain perfect coupling state or arbitrary coupling state easy by etching off the parton the waveguides or between the waveguides.

CONSTITUTION: Light of a wavelength λ is admitted from a waveguide 1 or 2; the relations between the propagation distance of the guided light and the quantity of transition of the light between the waveguides 1 and 2 are measured, and L0, Δβ are calculated by using the formula 1. Next, if the surface is slightly etched on the waveguide of larger Δβ by using ion etching or the like as shown in etching 3, phase matching conditions Δβ=0 is realized. Next, slots 4, 5 are scooped by etching between the waveguides 1 and 2 to separate the coupling part 6 from the noncoupling part. The length L of the coupling part is so selected as to satisfy the conditions of the formula 2. The etching 3 is applied in this way and slots 4, 5 are provided, whereby the arbitrary coupling state including 100% coupling may be easily obtained.