To attain impedance matching of input and output terminals, and to improve reflection characteristics in a high frequency range in a coplanar waveguide line with a back ground conductor.

A signal line conductor 13 and first and second ground conductors 11 and 12 are formed on the upper face side of a dielectric substrate 15, a third ground conductor 16 is formed on the lower face side, and a CPW line is formed of those conductors. The first and second ground conductors 11 and 12 are formed so as to be separated in an equal distance with the signal line conductor 13 interposed between them. The first, second, and third ground conductors 11, 12, and 16 are electrically connected through a first through hole 14 so that their potentials can be made equal. Moreover, the first and second ground conductors 11 and 12 and the third ground conductor 16 are electrically connected through a second through hole 17 formed like a long groove from input and output edge faces 15a and 15b of the dielectric substrate 15 to the neighborhood of the edge faces.