An air conditioner includes a purifying unit having at least one filter; a sensor unit including an air quality sensor; and a controller configured to: calculate a filter contamination level based on (i) data regarding indoor air quality measured by the air quality sensor during an operation of the air conditioner, and (ii) an operating time of the air conditioner; combine the calculated filter contamination level with a pre-stored filter contamination level to obtain a combined filter contamination level; and based on the combined filter contamination level satisfying at least one filter replacement condition, output filter replacement indicator information.