To employ a prefabricated overhead line installation method when an overhead transmission line with a jumper device is installed.

Transmission lines 7 are anchored by wedged clamps 3 and a jumper wire 9 is supported between the wedged clamps 3 on both the sides of a tower so as to sag freely to compose a jumper device 1B. When the ovrehead line is installed, after a prefabricated wire which is measured beforehand and marked so as to have a length obtained by preliminary precise measurement and calculation is extended, all the excessive length of the wire produced by the sag adjustment is drawn into the jumper device, and the part of the wire of a length which is not necessary to form the jumper wire into a predetermined form is deformed into a U-shape by plastic deformation on tightening the wire. In other words, a U-shaped excessive length 20 of the jumper wire is formed. As the excessive length of the jumper wire is absorbed by the jumper device, a problem that the sag adjustment of the span wires 8 has large influence upon the sag of the jumper wire 9 can be solved.