Flat tube heat exchangers as a plurality of single-row flat tube heat exchangers that are coupled to each other, each of the single-row flat tube heat exchangers comprising: flat tubes each having a rounded rectangular shape with a high aspect ratio in cross section, the flat tubes allowing a heat exchange medium to flow therein; and a plurality of plate-shaped fins in which the flat tubes in a state of being bent into U shapes having hairpin corners are inserted, the fins being brazed to the flat tubes in a direction perpendicular to the flat tubes, wherein the flat tubes are arranged at a predetermined pitch in a stage direction orthogonal to a row direction of the fins, a distance between fin ends at one side in the stage direction of the fins and a center in a thickness direction of the flat tubes is k·Dp and a distance between fin ends at the other end in the stage direction of the fins and the center in the thickness direction of the flat tubes is (1-k)·Dp, where Dp is a pitch of the flat tubes in the stage direction and k is a coefficient of Dp, and either 0 < k < 0.5 or 0.5 < k < 1, an odd-numbered one of the single-row flat tube heat exchangers is disposed in opposite orientation with respect to the stage direction to an even-numbered one of the single-row flat tube heat exchangers with regard to an air flow direction, and upper and lower ends of the odd-numbered one of the single-row flat tube heat exchangers are aligned with upper and lower ends of the even-numbered one of the single-row flat tube heat exchangers, the fins of the flat tube heat exchangers have surfaces on which a plurality of heat exchange accelerators (16, 17) are disposed, and characterized in that the odd-numbered one of the single-row flat tube heat exchangers and the even-numbered one of the single-row flat tube heat exchangers are disposed such that in a view from the air flow direction, the heat exchange accelerators (16, 17) are disposed only on a fin surface of the odd-numbered one of the single-row flat tube heat exchangers opposite to a fin surface of the even-numbered one of the single-row flat tube heat exchangers at which the heat exchange accelerators (16, 17) are disposed. flat tubes each having a rounded rectangular shape with a high aspect ratio in cross section, the flat tubes allowing a heat exchange medium to flow therein; and a plurality of plate-shaped fins in which the flat tubes in a state of being bent into U shapes having hairpin corners are inserted, the fins being brazed to the flat tubes in a direction perpendicular to the flat tubes, wherein the flat tubes are arranged at a predetermined pitch in a stage direction orthogonal to a row direction of the fins, a distance between fin ends at one side in the stage direction of the fins and a center in a thickness direction of the flat tubes is k·Dp and a distance between fin ends at the other end in the stage direction of the fins and the center in the thickness direction of the flat tubes is (1-k)·Dp, where Dp is a pitch of the flat tubes in the stage direction and k is a coefficient of Dp, and either 0 < k < 0.5 or 0.5 < k < 1, an odd-numbered one of the single-row flat tube heat exchangers is disposed in opposite orientation with respect to the stage direction to an even-numbered one of the single-row flat tube heat exchangers with regard to an air flow direction, and upper and lower ends of the odd-numbered one of the single-row flat tube heat exchangers are aligned with upper and lower ends of the even-numbered one of the single-row flat tube heat exchangers, the fins of the flat tube heat exchangers have surfaces on which a plurality of heat exchange accelerators (16, 17) are disposed, and characterized in that the odd-numbered one of the single-row flat tube heat exchangers and the even-numbered one of the single-row flat tube heat exchangers are disposed such that in a view from the air flow direction, the heat exchange accelerators (16, 17) are disposed only on a fin surface of the odd-numbered one of the single-row flat tube heat exchangers opposite to a fin surface of the even-numbered one of the single-row flat tube heat exchangers at which the heat exchange accelerators (16, 17) are disposed.