A water electrolysis system includes a water electrolytic stack, a water reservoir connected to the water electrolytic stack to supply water to the water electrolytic stack, a water circulation pump supplying the water in the water reservoir to the electrolytic stack; and a control unit configured to, during an operation stoppage of the electrolysis system, control the driving of the water circulation pump to convert the water in the electrolytic stack from an acidic condition to a neutral condition and to regulate a unit cell voltage of the electrolytic stack to a voltage such that an electrolysis reaction does not occur and a chemical state of an anode catalyst is stably maintained.