PURPOSE: To prevent a device from being used while forgetting to exchange water and to provide a simple device in a water exchange work in an air purifier used chiefly in a general household or an office, etc.

CONSTITUTION: The air purifier automatically operates a water discharge pump 21 when after 24 hours supplying water supply, and the water in a water tank 7 is discharged to the tank 6. Besides since the water is also automatically discharged with the water discharge pump 21 even when the water quantity attains a minimum during the operation, use of the soiled water or the water left for many hours is avoided.