PURPOSE: To keep electric power loss small by using a high AC voltage power source, applying the high AC voltage which is the output thereof to a pulse wave forming element via a rectifier and charging said element from zero voltage up to the voltage of the AC crest value.

CONSTITUTION: The high AC voltage which is the output of a high AC voltage power source 17 consisting of an AC power source 15 and a boosting transformer 16 is applied to both terminals 19, 20 of a high-voltage pulse wave forming element 5 via conductors 2, 3 and a rectifier 18. In the conducting direction of the rectifier 18, the high-voltage pulse waveform forming element is charged in the negative half period of the high AC voltage efficiently up to its crest value -Vm. Then at the prescribed point of the time in the half period when the rectifier 18 is in a blocking state, a high-speed synchronizing switch element 21 of external control type is conducted. Thus the voltage 5 at both terminals is applied between the core 22 of a high-voltage coaxial cable for transmission and an external conductor 24 through conductors 7, 8 and the switch elements 21, 23.