PURPOSE: To offer a feeder line with a low loss by selecting the surface roughness Ra of both sides of a strip conductor and a smoothing face side of two ground conductors to be both 0.3μm or below and arranging the smoothing face of the two ground conductors opposite to the strip conductor.

CONSTITUTION: The upper and lower smoothing faces of ground conductors 2, 3 are arranged opposite respectively to a strip conductor 1. The surface roughness of the smoothing face of the ground conductors and both sides of the strip conductor is selected to be the center line average roughness Ra of 0.3μm stipulated by the JIS standard B0601 or below and preferably 0.2μm or below. The resistance of the strip conductor 1 is decreased in the feeding line of the suspended triplate system obtained through the constitution above more than that of the feeding line of the suspended tri-plate system of a conventional structure.