The electrolysis system of the present invention has a water electrolysis unit equipped with a water electrolysis cell using a solid polymer electrolyte membrane as its diaphragm, a DC power source for supplying the water electrolysis cell with a DC current, and a mitigation unit for mitigating, when a counter electromotive force or abrupt potential fluctuation occurs in the water electrolysis cell, the counter electromotive force or the potential fluctuation. The mitigation unit is preferably a coil+diode (A) connected in parallel between the DC power source and the water electrolysis cell or a diode (B) connected between the DC power source and the water electrolysis cell. The electrolysis system is preferably equipped further with a lowering rate controller for controlling the lowering rate of a current supplied from the DC power source to the water electrolysis cell.