The LSE mode, which is a parasitic mode, can be effectively suppressed by a simple structure, and resultantly an improvement for small scale and lightweight is promoted. An NRD guide for propagating an electromagnetic wave comprises parallel conductor plates with a spacing of below 1/2 of the wavelength and a dielectric line (1) sandwiched between the parallel conductor plates. A metal body (3) is disposed near the dielectric line (1) so as to suppress the LSE mode. The metal body (3) can have a given shape such as a circular disk, an elliptic disk, or a prism. While maintaining the distance d between the metal body (3) and the dielectric line (1), the distance d is varied. Thus, the phase constant difference can be controlled.
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