A refractive index n of a dielectric material is larger than a refractive index of the outside in a lateral direction X and/or a vertical direction Y perpendicular to an electromagnetic wave travelling direction Z, the inside of a waveguide-path has slow electromagnetic wave propagation velocity, compared to an area on the outside, the maximum dimension in the lateral direction and/or the vertical direction of the waveguide-path has a dimension which is specified by a formula below. The formula is: tan(ksa/2)=kf/ks, or tan(ksa/2)=−ks/kf. Here, ks: propagation constant of an electromagnetic wave low-speed area, kf: propagation constant of an electromagnetic wave high-speed area, and a: maximum dimension in the X direction and/or the Y direction of the waveguide-path.