To improve the efficiency of a reduction reaction. A power source applies a voltage to an oxidation electrode immersed in an aqueous solution in an oxidation tank and a reduction electrode immersed in an aqueous solution in a reduction tank, the voltage having a voltage value that changes with a predetermined cycle to be a voltage value at which ions can be desorbed from a surface of the oxidation electrode and a surface of the reduction electrode during one cycle of the voltage change. The frequency of the voltage is set within a range of 10 Hz to 1 kHz.