To provide a water electrolysis system constituted in such a manner that, even when the most adequate working point, hence the maximum output of a solar battery fluctuates, efficient operation is made possible by utilizing this maximum output.

The water electrolysis system 1 has a water electrolyzer 4, the solar battery 2 which is a power source of this water electrolyzer 4 and a DC/DC converter 5 which converts the maximum output to the current and voltage corresponding to the IV characteristic of the water electrolyzer 4 and inputs the same to the water electrolyzer 4 by converting the current and voltage of a part of the max. output of the solar battery 2.