A heat exchanger comprising: a plurality of flat pipes arranged in an up-down direction in a state in which flat surfaces are directed upward and downward, the plurality of flat pipes extending in a fin stacking direction intersecting an air flow direction; and a plurality of heat transfer fins each including a plurality of cutouts into which the plurality of flat pipes are inserted, the plurality of cutouts extending from a leeward side toward a windward side in the air flow direction, a plurality of fin main parts each formed between the cutouts that are adjacent to each other in the up-down direction, a fin windward part extending on the windward side of the plurality of cutouts in the air flow direction continuously with the plurality of fin main parts, and a fin collar part extending from a peripheral portion of each of the cutouts toward one side in the fin stacking direction, the plurality of heat transfer fins being stacked in the fin stacking direction, wherein a portion of each of the fin collar parts along a flat upper surface, which is the flat surface on an upper side of the flat pipe, is a fin collar upper portion, the fin collar upper portion and the flat upper surface forming therebetween a gap extending in the air flow direction, the fin collar upper portion including a window portion through which a space that is between the fin main parts adjacent to each other in the fin stacking direction and that is on an upper side of the fin collar upper portion and the gap are in communication with each other characterized in that the fin collar upper portion includes a flare portion having a tip that curves upward into an R-shape, wherein the gap is formed between the flare portion and the flat upper surface, and wherein the window portion is formed in the flare portion. a plurality of flat pipes arranged in an up-down direction in a state in which flat surfaces are directed upward and downward, the plurality of flat pipes extending in a fin stacking direction intersecting an air flow direction; and a plurality of heat transfer fins each including a plurality of cutouts into which the plurality of flat pipes are inserted, the plurality of cutouts extending from a leeward side toward a windward side in the air flow direction, a plurality of fin main parts each formed between the cutouts that are adjacent to each other in the up-down direction, a fin windward part extending on the windward side of the plurality of cutouts in the air flow direction continuously with the plurality of fin main parts, and a fin collar part extending from a peripheral portion of each of the cutouts toward one side in the fin stacking direction, the plurality of heat transfer fins being stacked in the fin stacking direction, wherein a portion of each of the fin collar parts along a flat upper surface, which is the flat surface on an upper side of the flat pipe, is a fin collar upper portion, the fin collar upper portion and the flat upper surface forming therebetween a gap extending in the air flow direction, the fin collar upper portion including a window portion through which a space that is between the fin main parts adjacent to each other in the fin stacking direction and that is on an upper side of the fin collar upper portion and the gap are in communication with each other characterized in that the fin collar upper portion includes a flare portion having a tip that curves upward into an R-shape, wherein the gap is formed between the flare portion and the flat upper surface, and wherein the window portion is formed in the flare portion.