An air conditioner includes a main body including a body fan, a heat exchanger, and a body housing. A ventilation hose connects the main body to outdoor air. A ventilation module is attached to an outer side of the body housing, and includes a ventilation fan. The ventilation module switches between an exhaust mode in which indoor air is exhausted to the outdoors through the ventilation hose and an air supply mode in which outdoor air is supplied to the main body through the ventilation hose.