An indoor unit of an air conditioner comprising: a housing comprising a front panel provided with at least one opening and a rear panel coupled to a rear side of the front panel; at least one discharge outlet exposed to a front of the front panel through the opening; at least one suction inlet formed in the rear panel at a position corresponding to the discharge outlet; at least one heat exchanger disposed in front 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; 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; and a diffuser disposed at a front side of the diagonal flow fan, wherein the diffuser comprises: a circular disc plate; and a grille coupled to an outer circumferential surface of the circular disc plate to form the discharge outlet between the grille and the circular disc plate, characterized in that the indoor unit further comprises a drive motor coupled to a rear surface of the circular disc plate, a rotating shaft of the drive motor arranged facing the rear panel of the housing, and a duct coupled to a rear surface of the diffuser to form a flow passage allowing the air suctioned by the diagonal flow fan to be discharged to the discharge outlet therethrough; wherein the diagonal flow fan comprises: a hub coupled to the rotating shaft of the drive motor; and a plurality of blades coupled to an outer circumferential surface of the hub; wherein the hub and the duct are arranged to guide the air towards the at least one discharge outlet. a housing comprising a front panel provided with at least one opening and a rear panel coupled to a rear side of the front panel; at least one discharge outlet exposed to a front of the front panel through the opening; at least one suction inlet formed in the rear panel at a position corresponding to the discharge outlet; at least one heat exchanger disposed in front 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; 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; and a diffuser disposed at a front side of the diagonal flow fan, wherein the diffuser comprises: a circular disc plate; and a grille coupled to an outer circumferential surface of the circular disc plate to form the discharge outlet between the grille and the circular disc plate, characterized in that the indoor unit further comprises a drive motor coupled to a rear surface of the circular disc plate, a rotating shaft of the drive motor arranged facing the rear panel of the housing, and a duct coupled to a rear surface of the diffuser to form a flow passage allowing the air suctioned by the diagonal flow fan to be discharged to the discharge outlet therethrough; wherein the diagonal flow fan comprises: a hub coupled to the rotating shaft of the drive motor; and a plurality of blades coupled to an outer circumferential surface of the hub; wherein the hub and the duct are arranged to guide the air towards the at least one discharge outlet. a circular disc plate; and a grille coupled to an outer circumferential surface of the circular disc plate to form the discharge outlet between the grille and the circular disc plate, characterized in that the indoor unit further comprises a drive motor coupled to a rear surface of the circular disc plate, a rotating shaft of the drive motor arranged facing the rear panel of the housing, and a duct coupled to a rear surface of the diffuser to form a flow passage allowing the air suctioned by the diagonal flow fan to be discharged to the discharge outlet therethrough; wherein the diagonal flow fan comprises: a hub coupled to the rotating shaft of the drive motor; and a plurality of blades coupled to an outer circumferential surface of the hub; wherein the hub and the duct are arranged to guide the air towards the at least one discharge outlet. a hub coupled to the rotating shaft of the drive motor; and a plurality of blades coupled to an outer circumferential surface of the hub; wherein the hub and the duct are arranged to guide the air towards the at least one discharge outlet.