To reduce the quantity of consumed power for electrolysis and reduce the running cost by this, by branching a material water intake pipe for chlorine generation, which supplies a chlorine generator with material water, from a circulation water line between a condenser and a cooling tower, and using blow water as material water.

A material water intake pipe 13A for chlorine generation is one whose connection position is changed from that of conventional one, and it is branched from a pipe section which leads to a cooling tower 5 from the condenser 4 of a circulating water pipe 9, and with this blow water is supplied as material water to a chlorine generator 6 after being boosted with a booster pump 14. In this case, the moisture of circulating water evaporates in the cooling tower 5, and when the concentration of chlorine goes higher, the consumed power required for the electrolysis in the chlorine generator 6 becomes small. Accordingly, the chlorine can be produced and returned to the line with less consumed power as compared with a conventional art, so the running cost can be reduced.