A fuel cell power plant comprises a water circulation passage (5) which circulates the cooling water in a fuel cell stack (4). Sodium hydroxide which lowers the melting point of water is mixed with water in the water circulation passage (5) to prevent freezing of the water in the water circulation passage (5). Electrodes (11, 12) are disposed in the water. When the fuel cell stack (4) is running, a voltage is applied between the electrodes (11, 12) such that the positive electrode (11) attracts sodium ions, thereby removing sodium ions from the cooling water supplied to the fuel cell stack (4). When the fuel cell stack (4) stops running, sodium ions which were attracted to the positive electrode (11) are made to diffuse into the cooling water by ceasing to apply the voltage to the electrodes (11, 12).