An operation mode of an air cleaner is changed according to the degree of indoor air contamination and an indoor space condition, and one or more filters of the air cleaner can be easily cleaned. The air cleaner includes a housing, an inlet located at a lower part of the housing, a filter assembly, an outlet, and a brush unit. The filter assembly is located above the inlet, and a diameter of the filter assembly is gradually shortened in the direction from an upper part of the filter assembly to a lower part of the filter assembly. The discharge port is located above a second filter assembly. The brush unit is used to eliminate contaminants from an outer surface of the filter assembly.