body 8 through the down corner 12 the alkali ingredient i e bicarbonate or carbonate contained in the supply water undergoes thermal decomposition in the boiler body 8 through heating and produces a hydroxide and is concentrated to increase the ph of the boiler water at the same time the silica which is a corrosion suppressing ingredient contained in the supply water is also concentrated during combustion in the boiler 2 to maintain the ph of the boiler water in the range of 11 to 12 opening closing control is performed on the boiler water gathering valve 26 to thereby periodically discharge from within the boiler body 8 alkaline boiler water in an amount corresponding to 10 to 20 of the supply water amount that is a portion of the alkaline boiler water is supplied to the filter portion 27 through the boiler water gathering channel 25 at the filter portion 27 sludge or the like contained in the boiler water is removed by filtration to thereby clean the boiler water the filtered boiler water is supplied to the supply water tank 15 through the boiler water supply channel 28 by the steam pressure within the boiler body 8 inside the supply water tank 15 the supply water and the boiler water collide to.