PURPOSE: To reduce the insertion loss by melting and bonding a center conductor to one face of an alumina base and a ground conductor made of a material not alloyed with solder to other face by means of a melting layer respectively made of a low melting point low loss material.

CONSTITUTION: The center conductor 8 and the ground conductor 9 are molten and fixed to both faces of the alumina base 1 via melting layers 7a, 7b made of a low melting point low loss material such as a low melting point glass. Since the type of material of the ground conductor 9 is made of a metal not alloyed to a solder member such as copper in soldering the microstrip line to the ground conductor 6 in this way, an alloy layer is hardly formed. Further, since the conductors 8, 9 are molten directly to the base 1, the metallic vapor-deposition layer is omitted.