A top phase transmission line and bottom phase transmission line are stretched between support steel towers so that the bottom phase transmission line is positioned below the top phase transmission line. Each of the transmission lines has at least two conductors, which conductors are arranged to be substantially parallel and are separated by a substantially constant distance by interconductor spacers. An intermediate phase transmission line may be arranged between the top phase transmission line and bottom phase transmission line as well. Such transmission lines are multiconductor transmission lines. In these multiconductor transmission lines, the conductor positioned at the left side in the top phase transmission line and the conductor positioned at the right side in the intermediate phase transmission line or bottom phase transmission line directly below it are connected by a first interphase spacer made of an insulator. Further, the conductor positioned at the right side in the top phase transmission line and the conductor positioned at the left side in the intermediate phase transmission line or bottom phase transmission line directly below it are connected by a second interphase spacer made of an insulator. By this, galloping in the transmission lines can be suppressed.