A reversible fan comprising: an impeller configured to be rotatable about a rotation axis and having a plurality of blades radially attached to the perimeter of the impeller, wherein with rotation of the blades, the impeller is able to provide a current of air either in a normal direction or in a reverse direction, where a direction where air is discharged when the impeller rotates in a normal rotation direction (n-direction) is the normal direction and where a direction where wind is discharged when the impeller rotates in a reverse rotation direction (r-direction) is the reverse direction; a motor configured to rotate the impeller, said motor being assembled in the impeller in such a manner as to be placed on the normal direction side relative the impeller; a base portion supporting the motor and being provided in such manner as to cover the normal direction side of the motor, a tubular frame housing the impeller, the motor and the base portion; characterized by a fin (10; 10A-H) protruding inward in a radial direction from an inner peripheral surface (53, 53A-53H) of the frame; wherein the fin includes a first curved surface, a second curved surface (12; 12AH), and an edge, the first curved surface forms a surface in the normal rotation direction (n-direction) of the fin, and protrudes in the normal rotation direction, the second curved surface forms a surface in the reverse rotation direction (r-direction) of the fin, and is recessed in the normal rotation direction, the edge connects the first and second curved surfaces, and wherein the fin is formed in the form of a thin plate including the first curved surface as a front, outer side of the fin and the second curved surface as a back, inner side of the fin and providing an opening portion that is open to a normal rotation direction (n-direction) of the impeller and open to the current of air in the normal direction when the impeller rotates in the normal rotation direction, such that a separation space (14; 14A-H) is formed between the second curved surface of the fin and the inner peripheral surface (53, 53A-53H) of the frame, the separation space being configured in such a manner that, when the impeller rotates in the reverse rotation direction, a part of the current of air in the reverse direction is separated to flow in the separation space at an end of the edge in the reverse rotation direction. an impeller configured to be rotatable about a rotation axis and having a plurality of blades radially attached to the perimeter of the impeller, wherein with rotation of the blades, the impeller is able to provide a current of air either in a normal direction or in a reverse direction, where a direction where air is discharged when the impeller rotates in a normal rotation direction (n-direction) is the normal direction and where a direction where wind is discharged when the impeller rotates in a reverse rotation direction (r-direction) is the reverse direction; a motor configured to rotate the impeller, said motor being assembled in the impeller in such a manner as to be placed on the normal direction side relative the impeller; a base portion supporting the motor and being provided in such manner as to cover the normal direction side of the motor, a tubular frame housing the impeller, the motor and the base portion; characterized by a fin (10; 10A-H) protruding inward in a radial direction from an inner peripheral surface (53, 53A-53H) of the frame; wherein the fin includes a first curved surface, a second curved surface (12; 12AH), and an edge, the first curved surface forms a surface in the normal rotation direction (n-direction) of the fin, and protrudes in the normal rotation direction, the second curved surface forms a surface in the reverse rotation direction (r-direction) of the fin, and is recessed in the normal rotation direction, the edge connects the first and second curved surfaces, and wherein the fin is formed in the form of a thin plate including the first curved surface as a front, outer side of the fin and the second curved surface as a back, inner side of the fin and providing an opening portion that is open to a normal rotation direction (n-direction) of the impeller and open to the current of air in the normal direction when the impeller rotates in the normal rotation direction, such that a separation space (14; 14A-H) is formed between the second curved surface of the fin and the inner peripheral surface (53, 53A-53H) of the frame, the separation space being configured in such a manner that, when the impeller rotates in the reverse rotation direction, a part of the current of air in the reverse direction is separated to flow in the separation space at an end of the edge in the reverse rotation direction.