A heat exchanger comprising: a fin group; and a tube group which is mounted to and penetrates through the fin group and through which a heat exchange medium flows to exchange heat with air for air conditioning, wherein the fin group includes plate fins each of which is shaped as a flat polygonal plate having corners including a first corner whose internal angle (θ) is an acute angle and each of which includes first and second sides and forming the first corner, the plate fins being arranged such that respective plate surfaces of the plate fins face one another across gaps through which the air for air conditioning passes and such that the air for air conditioning flows in a first direction along the first side, the tube group includes heat transfer tubes each of which extends in the first direction and meanders in such a manner as to cross the first direction, each of the heat transfer tubes includes fin-mounted portions spaced in a direction along the first side and a direction along the second side, the fin-mounted portions being mounted to and penetrating through the fin group at locations in the fin group, and the fin-mounted portions mounted to each of the plate fins are arranged such that between first fin-mounted portions adjacent to one another in the first direction, there are second fin-mounted portions adjacent to the first fin-mounted portions in a direction extending along the plate surface of the plate fin and orthogonal to the first direction, characterized in that the plate fins are arranged such that the first corner of each of the plate fins is located towards upstream in the first direction; the plate fins are quadrilateral having the shape of a parallelogram; in each of the plate fins, an internal angle (θ) of a second corner opposite to the first corner is an acute angle; each of the plate fins includes third and fourth sides and forming the second corner, the third side being opposite to the first side, the fourth side being opposite to the second side; the first side extending in the first direction; the heat exchanger further comprises an outlet for the air for air conditioning, the outlet being located on a side surface of the fin group along which the fourth sides of the plate fins are aligned, the outlet sloping downward from upstream to downstream in the first direction; each of the heat transfer tubes has a cross-section whose outer peripheral shape is elliptical; on each of the plate fins, the second fin-mounted portions are offset from the first fin-mounted portions in the direction orthogonal to the first direction and are located between the first fin-mounted portions in the first direction; and the heat transfer tubes are arranged such that a major axis of the elliptical shape of the cross-section extends in the first direction. a fin group; and a tube group which is mounted to and penetrates through the fin group and through which a heat exchange medium flows to exchange heat with air for air conditioning, wherein the fin group includes plate fins each of which is shaped as a flat polygonal plate having corners including a first corner whose internal angle (θ) is an acute angle and each of which includes first and second sides and forming the first corner, the plate fins being arranged such that respective plate surfaces of the plate fins face one another across gaps through which the air for air conditioning passes and such that the air for air conditioning flows in a first direction along the first side, the tube group includes heat transfer tubes each of which extends in the first direction and meanders in such a manner as to cross the first direction, each of the heat transfer tubes includes fin-mounted portions spaced in a direction along the first side and a direction along the second side, the fin-mounted portions being mounted to and penetrating through the fin group at locations in the fin group, and the fin-mounted portions mounted to each of the plate fins are arranged such that between first fin-mounted portions adjacent to one another in the first direction, there are second fin-mounted portions adjacent to the first fin-mounted portions in a direction extending along the plate surface of the plate fin and orthogonal to the first direction, characterized in that the plate fins are arranged such that the first corner of each of the plate fins is located towards upstream in the first direction; the plate fins are quadrilateral having the shape of a parallelogram; in each of the plate fins, an internal angle (θ) of a second corner opposite to the first corner is an acute angle; each of the plate fins includes third and fourth sides and forming the second corner, the third side being opposite to the first side, the fourth side being opposite to the second side; the first side extending in the first direction; the heat exchanger further comprises an outlet for the air for air conditioning, the outlet being located on a side surface of the fin group along which the fourth sides of the plate fins are aligned, the outlet sloping downward from upstream to downstream in the first direction; each of the heat transfer tubes has a cross-section whose outer peripheral shape is elliptical; on each of the plate fins, the second fin-mounted portions are offset from the first fin-mounted portions in the direction orthogonal to the first direction and are located between the first fin-mounted portions in the first direction; and the heat transfer tubes are arranged such that a major axis of the elliptical shape of the cross-section extends in the first direction.