A water electrolysis system 1000 comprises: a plurality of electrolysis stacks that are series-connected to a DC power source; a water conveyance pump 109 for supplying water to the entirety of the series-connected electrolysis stacks 101; a large-sized cooler 110 for adjusting the temperature of water supplied to the entirety of the electrolysis stacks 101; water flow rate adjusting valves 111 and small-sized coolers 112 that are provided in water supplying pipes between the electrolysis stacks 101 and the water conveyance pump 109 or the large-sized cooler 110; and a control device for controlling the water conveyance pump 109, the large-sized cooler 110, the small-sized coolers 112, and the water flow rate adjusting valves 111. The control device adjusts the amounts of water supplied to the respective electrolysis stacks 101 by means of the water rate adjusting valves 111, and adjusts the temperature of water supplied to the respective electrolysis stacks 101 by means of the small-sized coolers 112.