PURPOSE: To produce photo coupling with thin film optical transmission path, suppress undesired radiation, and increase the photo coupling efficiency, by forming grating of more than two layers in the thin film optical transmission path, and disposing other optical circuits in this grating part.

CONSTITUTION: Gratings of two layers 3a, 3b are formed in a thin film optical transmission path 3. Refractive index of air 1, substrate 2 and thin film optical transmission path 3 is assumed to n1, n2, n3, respectively; amplitude of gratings 3a, 3b to a1, a2, and phases thereof to 1, 2; and thickness of thin film optical transmission path 3 to d. In this state, the light of which wavelength is λ=0.85μm is propagated in the thin film optical transmission path 3 in the direction of arrow. Suppose n1=1.0, n2=1.515, n3=1.75, d/λ=1.55, r/k0=0.4, a1/λ=1.86×10-2, a2/λ=3.76×10-2, 1=0, 2=2.70, then radiation pattern as shwon in the drawing may be obtained. {a1, a2 are crest values of grating, k0=2π/λ, r=βA-θ(βA is propagation constant in direction Z of wave guide mode, θ is phase constant of grating).} Namely, undesired radiation may be suppressed, and the radiation in the direction of about 70° may be increased.