PURPOSE: To provide forced line charging reliability equal to that in application of a gap type lightning arrester insulator, to suppress potential increase in a steel tower after lightning, and to suppress an open/close surge so as to reduce an insulating distance.

CONSTITUTION: Multiple steel towers 2 are stood between substations 1, and three-phase two-wire transmission lines 3α-3γ, 4α-4γ are supported in each steel tower 2 via supporting insulators 5. In this transmission line path, twist system steel towers 6 are arranged at the predetermined intervals, and in the transmission lines 3α-3γ, 4α-4γ. each of upper, intermediate, lower phases is transferred mutually. In the steel tower 2 arranged in the lightning high frequency position in each of sections A, B, γ divided by the twist system steel towers 6, a gap type lightning arrester insulator device 8 is applied for the intermediate phase in which a lightning current waveheight value in the lightning frequency takes the greatest value, while for the upper and lower phases, gapless type lightning arrester insulator devices 9 are applied.