Abstract: An indoor unit of an air conditioner which has a simple construction and can be easily assembled. The indoor unit has a housing including a front frame and a rear frame coupled to the front frame, an evaporator installed in the housing for generating a chilled air, a blow fan for blowing the chilled air generated by the evaporator, a motor for rotating the blow fan, a drain pan for receiving and discharging a defrosted water dropped from the evaporator, ribs which are coupled with the drain pan for rotatably fixing the blow fan to the rear frame, and a hook assembly for detachably securing the drain pan to the rear frame. The indoor unit does not require a separate device for securing the blow fan and the drain pan to the rear wall of the housing, so the number of elements and assembling steps are reduced.