PURPOSE: To control effectively the water feeding of a wet-type dust collector conforming to a restriction for soot and dust, by discharging circulating suspended matters together with a drain, and feeding water, corresponding in quantity to the discharging water quantity to a water circulating line, and controlling the feeding quantity of a pH adjusting agent.

CONSTITUTION: After catching dust and mist of sulfuric acid at the front and rear chambers 12 and 13 of a wet-type dust collector 11, a dust collecting waste water is returned to a circulating water tank 14 and is circulated to the collector 11 after being filtered by a auto-strainer 16. A drain, containing solid matters separated by the strainer 16, is discharged out of the system. Further, an alkaline pH adjusting liquid is injected to a circulating water tank 14 through a pH controlling device 18 to neutralize the circulating water into a prescribed pH. On the other hand, a fresh water is replenished to the tank 14 as the descreased portion of circulating water caused by the drain discharge. Further, the fresh water is intermittently fed as a washing water to the rear chamber 13 of the dust collector 11 by an intermittently acting pump 20.