A heat-exchanger, comprising: a first flat-tube group including a plurality of flat tubes each having a pipe axis, the plurality of flat tubes included in the first flat-tube group being arranged in such a manner that the pipe axes of the plurality of flat tubes included in the first flat-tube group are arranged in parallel to each other, one of the plurality of flat tubes included in the first flat-tube group being a first flat tube; a second flat-tube group provided adjacent to the first flat-tube group, the second flat-tube group including a plurality of flat tubes each having a pipe axis, the plurality of flat tubes included in the second flat-tube group being arranged in such a manner that the pipe axes of the plurality of flat tubes included in the second flat-tube group are arranged in parallel to each other, one of the plurality of flat tubes included in the second flat-tube group being a second flat tube; a fin provided to the first flat-tube group and the second flat-tube group, the fin including a first portion connecting an end of a longitudinal axis of the first flat tube in a section perpendicular to the pipe axis of the first flat tube and an end of a longitudinal axis of the second flat tube in a section perpendicular to the pipe axis of the second flat tube, characterized in that the fin includes a second portion joined to a side wall of the first flat tube, and a third portion joined to a side wall of the second flat tube, at the second portion, one face of a plate of the fin being joined to the first flat tube, at the third portion, an other face of the plate being joined to the second flat tube. a first flat-tube group including a plurality of flat tubes each having a pipe axis, the plurality of flat tubes included in the first flat-tube group being arranged in such a manner that the pipe axes of the plurality of flat tubes included in the first flat-tube group are arranged in parallel to each other, one of the plurality of flat tubes included in the first flat-tube group being a first flat tube; a second flat-tube group provided adjacent to the first flat-tube group, the second flat-tube group including a plurality of flat tubes each having a pipe axis, the plurality of flat tubes included in the second flat-tube group being arranged in such a manner that the pipe axes of the plurality of flat tubes included in the second flat-tube group are arranged in parallel to each other, one of the plurality of flat tubes included in the second flat-tube group being a second flat tube; a fin provided to the first flat-tube group and the second flat-tube group, the fin including a first portion connecting an end of a longitudinal axis of the first flat tube in a section perpendicular to the pipe axis of the first flat tube and an end of a longitudinal axis of the second flat tube in a section perpendicular to the pipe axis of the second flat tube, characterized in that the fin includes a second portion joined to a side wall of the first flat tube, and a third portion joined to a side wall of the second flat tube, at the second portion, one face of a plate of the fin being joined to the first flat tube, at the third portion, an other face of the plate being joined to the second flat tube.