An evaporator 1 comprises a heat exchange core 4 having heat exchange tubes 12 in groups 13, a refrigerant inlet header 5 and a refrigerant outlet header 6 which are arranged toward one end of each of the heat exchange tubes 12, and a refrigerant inflow header 9 and a refrigerant outflow header 11 which are arranged toward the other end of each heat exchange tube 12. The outflow header 11 has its interior divided by a flow dividing control wall 52 into two spaces 11a, 11b arranged one above the other. The inflow header 9 and the lower space 11b of the outflow header 11 are held in communication each at one end thereof. The control wall 52 has a plurality of refrigerant passing holes 53 arranged at a spacing longitudinally thereof.