To provide a method for preventing fungi on a room air conditioner, capable of evenly impregnating all places of a room and a filter in an indoor machine of the room air conditioner with an antibacterial and antifungal agent by sucking the evaporated gas of the antibacterial and antifungal agent from the suction opening of the indoor machine in the room of a general home and subsequently blowing out the gas from a blowout opening.

In the fungus-preventing tool of a room air conditioner, a gas generated from a volatile antibacterial and antifungal formulation directly or indirectly mounted on a place near to a suction opening 2 of the indoor machine 1 of the air conditioner is sucked from the suction opening 2, and subsequently blown out together with air from a blowout opening 4 of the indoor machine to impregnate a filter 3 of the indoor machine, a carpet or tatami mat (rush mat) in the room, and spaces behind or between furnitures with the gas.