PROBLEM TO BE SOLVED: To prevent re-volatilization of a chemical substance dissolved in cleaning water, in a system for circulating cleaning water for cleaning air by being brought into contact with the air.SOLUTION: An deaerator 9 is arranged in an intermediate part of piping 8 (circulation line of cleaning water) for connecting a circulation tank 2 to a sprinkler means 32. The deaerator 9 includes a water chamber 91 connected to the piping 8, an air chamber 93, and a deaerating membrane 94, that is, a gas permeable membrane arranged to separate the water chamber 91 from the air chamber 93, and further includes a vacuum pump (VP) 92 (depressurization means) for depressurizing the inside of the air chamber 93.
