A heat exchanger comprising: a plurality of plate-shaped fins stacked at predetermined fin pitches, each of the plate-shaped fins being provided with a plurality of notches arranged in a longitudinal direction of the plate-shaped fins; and a plurality of heat transfer tubes having a flattened shape, each of the heat transfer tubes being extended in a stacking direction of the plate-shaped fins and disposed in a corresponding one of the notches, each of the plurality of plate-shaped fins including, at circumferential portions thereof defining the notches, fin collars being in contact with an outer circumference of the heat transfer tube, each of the fin collars including at least one reflare section bent in a direction opposite to the side surface of the heat transfer tube being in contact with the fin collar, the plate-shaped fins being arranged at the predetermined fin pitches so that the reflare section of one of the adjoining plate-shaped fins comes into contact with another one of the adjoining plate-shaped fins, each of the heat transfer tubes having a plurality of flow passages formed inside thereof arranged in a long axis direction of the heat transfer tube, in each of the fin collars provided at the circumferential portions defining the same notch, a plurality of the reflare sections are provided along the long axis direction of corresponding one of the heat transfer tubes, characterized in that the at least one reflare section having a tip portion drawn apart from the adjoining plate-shaped fin, a distance between a first end portion serving as a windward side end portion of the heat transfer tube and the tip portion that is closest to the first end portion being longer than a distance between a second end portion (lb) serving as a leeward side end portion of the heat transfer tube and the tip portion that is closest to the second end portion. a plurality of plate-shaped fins stacked at predetermined fin pitches, each of the plate-shaped fins being provided with a plurality of notches arranged in a longitudinal direction of the plate-shaped fins; and a plurality of heat transfer tubes having a flattened shape, each of the heat transfer tubes being extended in a stacking direction of the plate-shaped fins and disposed in a corresponding one of the notches, each of the plurality of plate-shaped fins including, at circumferential portions thereof defining the notches, fin collars being in contact with an outer circumference of the heat transfer tube, each of the fin collars including at least one reflare section bent in a direction opposite to the side surface of the heat transfer tube being in contact with the fin collar, the plate-shaped fins being arranged at the predetermined fin pitches so that the reflare section of one of the adjoining plate-shaped fins comes into contact with another one of the adjoining plate-shaped fins, each of the heat transfer tubes having a plurality of flow passages formed inside thereof arranged in a long axis direction of the heat transfer tube, in each of the fin collars provided at the circumferential portions defining the same notch, a plurality of the reflare sections are provided along the long axis direction of corresponding one of the heat transfer tubes, characterized in that the at least one reflare section having a tip portion drawn apart from the adjoining plate-shaped fin, a distance between a first end portion serving as a windward side end portion of the heat transfer tube and the tip portion that is closest to the first end portion being longer than a distance between a second end portion (lb) serving as a leeward side end portion of the heat transfer tube and the tip portion that is closest to the second end portion.