PURPOSE: To reduce the size and cost of a power transmitting facility equipped with a power storing facility by interposing a DC circuit between AC systems.

CONSTITUTION: A power storing device 33 is incorporated in a DC system 32 connected between two AC systems 31 and 34 and a controller 22 which controls the device 33 to store electric power when the systems 31 and 34 are sound and to send or absorb electric power when an accident occurs in either one of the systems 31 and 34 is provided to the system 32. Since the facility 33 is incorporated in the system 32, no power converter, such as the inverter, etc., is required and the size and cost of a power transmitting facility can be reduced.