Disclosed is an air conditioner (AC) indoor unit including a housing installed on the ceiling and having an inlet and an outlet provided around the inlet and having a pair of straight sections facing each other and a pair of curved sections facing each other; a heat exchanger provided inside the housing and arranged in a main flow path between the inlet and the outlet; a blower fan configured to suck in air through the inlet, allow the air to exchange heat with the heat exchanger, and discharge the air through the outlet; and an auxiliary flow path guiding an auxiliary air current to change a direction of an air current discharged from the outlet. The direction of the discharged air current may be controlled by sucking in air around the outlet or blowing air to the periphery of the outlet through the auxiliary flow path without a blade.