A heat exchanger comprising: first and second horizontal header pipes (2, 3) arranged parallel, one over the other, at an interval from one another; a plurality of vertical flat tubes arranged with a predetermined pitch between the first and second header pipes, with vertical refrigerant passages provided inside the flat tubes communicating with insides of the header pipes (2, 3); and corrugated fins arranged between the flat tubes, wherein refrigerant flows in through the first header pipe, which, in use, is lower in position, and flows out through the second header pipe, which, in use, is higher in position, the corrugated fins comprise upwind-side corrugated fins whose fin surface has a downward slope toward a downwind side and downwind-side corrugated fins whose fin surface has an upward slope toward the downwind side, and downwind-side ends of the upwind-side corrugated fins and upwind-side ends of the downwind-side corrugated fins are kept in contact with ribs formed on side faces of the flat tubes such that a predetermined interval is formed between the upwind-side corrugated fins and the downwind-side corrugated fins, the predetermined interval being a gap with a width of 4 mm or less such that water droplets running down the upwind-side corrugated fins and water droplets running down the downwind-side corrugated fins meet and flow out through the gap without causing a bridging phenomenon, in which water forms a film by its surface tension. first and second horizontal header pipes (2, 3) arranged parallel, one over the other, at an interval from one another; a plurality of vertical flat tubes arranged with a predetermined pitch between the first and second header pipes, with vertical refrigerant passages provided inside the flat tubes communicating with insides of the header pipes (2, 3); and corrugated fins arranged between the flat tubes, refrigerant flows in through the first header pipe, which, in use, is lower in position, and flows out through the second header pipe, which, in use, is higher in position, the corrugated fins comprise upwind-side corrugated fins whose fin surface has a downward slope toward a downwind side and downwind-side corrugated fins whose fin surface has an upward slope toward the downwind side, and downwind-side ends of the upwind-side corrugated fins and upwind-side ends of the downwind-side corrugated fins are kept in contact with ribs formed on side faces of the flat tubes such that a predetermined interval is formed between the upwind-side corrugated fins and the downwind-side corrugated fins, the predetermined interval being a gap with a width of 4 mm or less such that water droplets running down the upwind-side corrugated fins and water droplets running down the downwind-side corrugated fins meet and flow out through the gap without causing a bridging phenomenon, in which water forms a film by its surface tension. upwind-side corrugated fins whose fin surface has a downward slope toward a downwind side and downwind-side corrugated fins whose fin surface has an upward slope toward the downwind side, and downwind-side ends of the upwind-side corrugated fins and upwind-side ends of the downwind-side corrugated fins are kept in contact with ribs formed on side faces of the flat tubes such that a predetermined interval is formed between the upwind-side corrugated fins and the downwind-side corrugated fins, the predetermined interval being a gap with a width of 4 mm or less such that water droplets running down the upwind-side corrugated fins and water droplets running down the downwind-side corrugated fins meet and flow out through the gap without causing a bridging phenomenon, in which water forms a film by its surface tension.