To attain high reliability of bonding of a dielectric line to parallel planar plate conductors and small variations in the adhesive strength of the line, and attain excellent transmission characteristics for a high-frequency signal, even if the line is used in a severe environment in which temperature changes widely and a vibration or mechanical shock is applied easily.

In a non-radiating dielectric line in which a dielectric line 2 through which a high-frequency signal is transmitted is provided between a parallel planar conductors 1, 3 arranged at an interval of 1/2 or less of the wavelength of a high-frequency signal, the line 2 is bonded on the internal surfaces of the conductors 1, 3, and the arithmetic mean roughness Ra of the surface of the waveguide 2 to 0.02 μm≤Ra≤20 μm. Also, A/L is 1/50 or lower, where the length of the line 2 is L and the maximum height difference of the line 2 in the surface facing the internal surface of the conductors 1, 3 is A.