PURPOSE: To improve the charging/discharging efficiency by providing monitoring cells on both sides of the electrolyte inlet and outlet of a reaction cell, and controlling the supply of the electrolyte to the reaction cell in responce to the starting voltage measured by the monitoring cell.

CONSTITUTION: A monitoring cell 6 is provided on the electrolyte inlet side of a reaction cell 1, while a monitoring cell 7 is provided on the electrolyte outlet side. A judgment circuit 8 judges whether the difference of starting electric power between the monitoring cells 6, 7 is maintained at the predetermined value or not. The circuit 8 judges that an excessive electrode reaction has occurred in the reaction cell 1, if the difference is larger than the prescribed value, and gives control signals to a pump drive inverter 9 so as to increase the supply of electryte to the reaction cell 1. On the contrary the circuit 8 gives control signals for lowering the pump revolutions of supply pumps 4, 5 to the inverter 9 to reduce the aforesaid supply if the defference is smaller than the value. It is thus possible to improve the charging/discharging efficiency.