Two transmission lines are formed adjacent to each other at spacing on an upper surface of a base insulating layer, and a ground conductor layer is formed on a lower surface of the base insulating layer. The ground conductor layer is arranged to be opposite to at least part of one transmission line and at least part of the other transmission line in a width direction of the two transmission lines. When a width of one transmission line, a width of the other transmission line, a spacing between the two transmission lines and a width of the ground conductor layer in an arbitrary cross section perpendicular to the two transmission lines are referred to as W1, W2, S, Wg, respectively, the width Wg of the ground conductor layer is set to satisfy relationship of Wg<(W1+W2+S) and S!<=0.8 Wg.