An indoor unit of an air conditioner having improved structures of suction, discharge and/or flow passages which may increase operation efficiency, reduce noises and realize a compact size. The indoor unit includes a housing comprising a front panel and a rear panel coupled to a rear portion of the front panel, at least one discharge outlet exposed to a front of the front panel, at least one suction inlet formed in the rear panel at a position corresponding to the discharge outlet, at least one heat exchanger disposed at a front portion of the suction inlet to absorb heat from air introduced through the suction inlet or transfer heat to the air introduced through the suction inlet, and at least one diagonal flow fan disposed between the heat exchanger and the discharge outlet to suction air passing through the heat exchanger and discharge the air through the discharge outlet.