To provide clean circulating water used in a room air conditioner by compounding the circulating water with a water-soluble organic acid, as an antibacterial agent for water, having a high sterilization effect, the air conditioner being the one having a water storage tank and a circulating apparatus for circulating water stored in the storage tank and the circulating water being brought into direct contact with room air to clean, humidify, and cool it.

For the purpose of an antibacterial treatment and sterilization of the circulating water for the room air conditioner, a nonvolatile acidic antibacterial agent containing lactic acid is charged into the circulating water to keep it clean and safe without contamination.