An air conditioner includes a main unit having an enclosure defining an air outlet. The enclosure contains a heat exchanger generating a cool air or a warm air forming airflow running out of the air outlet. A pair of fan units are disposed on the opposite sides of the air outlet. The fan units are configured to suck a room air and to blow the room air. The heat exchanger includes a rear section having a length larger than that of a front section in the longitudinal direction of the air outlet. The rear section extends into at least one of spaces respectively defined behind the fan units.