a water electrolysis stack including a membrane electrode assembly in which an ion exchange membrane is sandwiched between an anode and a cathode, and configured to carry out electrolysis of water;

a gas-liquid separator configured to separate hydrogen gas generated by electrolysis of the water by the water electrolysis stack, and the water; and

a hydrogen pressure boosting stack including a membrane electrode assembly in which a proton exchange membrane is sandwiched between an anode and a cathode, and configured to boost the pressure of the hydrogen gas that is separated by the gas-liquid separator;

the water electrolysis system further comprising:

a first power source device configured to adjust a voltage applied to the anode and the cathode of the water electrolysis stack, and to supply a current of a current command value between the anode and the cathode;

a second power source device configured to adjust a voltage applied to the anode and the cathode of the hydrogen pressure boosting stack, and to supply a current of the current command value between the anode and the cathode; and

a control device configured to control the first power source device and the second power source device;

wherein the control device comprises:

a first command generating unit configured to generate a first current command value, which is a constant current command value;

a second command generating unit configured to generate a second current command value, which is an undefined current command value, based on an amount of the hydrogen gas inside the gas-liquid separator; and

a command output unit configured to output the first current command value to one from among the first power source device and the second power source device, and to output the second current command value to another one from among the first power source device and the second power source device.