A water amount controlling method according to the present disclosure includes: opening a discharge valve that discharges a liquid from a water tank if an impurity concentration is higher than or equal to a first reference value, and opening a sluice gate that introduces the liquid to the water tank a predetermined period after opening the discharge valve, the first reference value being lower than a saturation concentration; throttling the discharge valve and the sluice gate if the impurity concentration is higher than or equal to a second reference value and the temperature is lower than or equal to a third reference value, the second reference value being lower than the first reference value; and closing the discharge valve if the impurity concentration is lower than the second reference value, and closing the sluice gate a predetermined period after closing the discharge valve.