A heat exchanger 1 comprises a heat exchange core 4 composed of heat exchange tubes 12 in groups 13 in the form of two rows arranged in parallel in the direction of flow of air through the exchanger, a refrigerant inlet header 5 disposed on the upper-end side of the tubes 12 and having one row of heat exchange tubes 12 joined thereto and a refrigerant outlet header 6 disposed in the rear of the inlet header 5 on the upper-end side of the tubes 12 and having the other row of heat exchange tubes 12 joined thereto. A cap 19 for closing an opening of the inlet header 5 at one end thereof is provided with a refrigerant inlet 37. The cap 19 has a lower edge defining the inlet 37 and provided with a guide 40 upwardly slanting inwardly of the inlet header 5.