An indoor unit (100, 200, 300, 400) of an air conditioner comprising: a housing including a front panel in which at least one opening is formed; at least one diffuser disposed at a position corresponding to the at least one opening and including a discharge outlet; a drive motor coupled to a rear surface of the diffuser; and a diagonal flow fan rotatably coupled to the drive motor, wherein the diffuser includes: a circular disc plate; and a grille disposed at an outer side of an outer circumferential surface of the circular disc plate in a radial direction of the circular disc plate to form the discharge outlet between the grille and the circular disc plate, wherein the grille includes a plurality of blade plates (121d, 421d) extending from the outer circumferential surface of the circular disc plate to an inner circumferential surface of the grille in the radial direction of the circular disc plate, and each blade plate (121d, 421d) is formed in a curved shape in a direction opposite to a direction of rotation of the diagonal flow fan, wherein each blade plate (121d, 421d) comprises a front portion facing in a front direction and a rear portion facing in a rear direction, and characterised in that the curvature of the rear portion of each blade plate (121d, 421d) is larger than the curvature of the front portion of each blade plate (121d, 421d). a housing including a front panel in which at least one opening is formed; at least one diffuser disposed at a position corresponding to the at least one opening and including a discharge outlet; a drive motor coupled to a rear surface of the diffuser; and a diagonal flow fan rotatably coupled to the drive motor, wherein the diffuser includes: a circular disc plate; and a grille disposed at an outer side of an outer circumferential surface of the circular disc plate in a radial direction of the circular disc plate to form the discharge outlet between the grille and the circular disc plate, wherein the grille includes a plurality of blade plates (121d, 421d) extending from the outer circumferential surface of the circular disc plate to an inner circumferential surface of the grille in the radial direction of the circular disc plate, and each blade plate (121d, 421d) is formed in a curved shape in a direction opposite to a direction of rotation of the diagonal flow fan, wherein each blade plate (121d, 421d) comprises a front portion facing in a front direction and a rear portion facing in a rear direction, and characterised in that the curvature of the rear portion of each blade plate (121d, 421d) is larger than the curvature of the front portion of each blade plate (121d, 421d). a circular disc plate; and a grille disposed at an outer side of an outer circumferential surface of the circular disc plate in a radial direction of the circular disc plate to form the discharge outlet between the grille and the circular disc plate, wherein the grille includes a plurality of blade plates (121d, 421d) extending from the outer circumferential surface of the circular disc plate to an inner circumferential surface of the grille in the radial direction of the circular disc plate, and each blade plate (121d, 421d) is formed in a curved shape in a direction opposite to a direction of rotation of the diagonal flow fan, wherein each blade plate (121d, 421d) comprises a front portion facing in a front direction and a rear portion facing in a rear direction, and characterised in that the curvature of the rear portion of each blade plate (121d, 421d) is larger than the curvature of the front portion of each blade plate (121d, 421d).