PURPOSE: To propagate only a mode limited by the transmission body structure on a substrate and to suppress all the unwanted dummy modes present on the substrate by forming a layer composed of a resistant material having a high loss on the other face of the substrate provided with a coplaner transmission line on one face.

CONSTITUTION: A coplaner transmission body structure 10 is provided with a substrate 12, and a coplaner transmission line 14 shown as a common transmission waveguide is formed on the surface. The substrate 12 is formed by a dielectric material such as alumina or sapphire. The coplaner transmission waveguide 14 is provided with a center strip 16 and two ground faces 18 and 20 arranged in parallel on the same plane as the center strip 16. The coplaner transmission line 14 decides the suitable electromagnetic mode of radiation propagated in the transmission line 14. On the opposite face of the substrate 12, a layer 22 is formed by the resistant material having the high loss such as nichrome or tantalum nitride. Thus, by using the resistant material having the high loss, especially, by using nichrome, a non-exciting electromagnetic mode propagated in the coplaner transmission body structure 10 can be sufficiently decreased.