An indoor unit of an air conditioner having an improved frame structure in which a refrigerant pipe that connects an outdoor unit and the indoor unit can be easily connected. The indoor unit of the air conditioner includes: a frame; a heat exchanger mounted on the frame and having a refrigerant pipe in which a refrigerant flows; a hanger plate mounted on a rear surface of the frame and fixing the frame onto a wall surface; and a holder plate mounted on a lower portion of the frame, fixing the frame and the hanger plate, and supported by a fixing member of the wall surface. The holder plate selectively opens/closes the lower portion of the frame so that a pipe connection operation can be easily performed.