The evaporator includes the upper side and lower header members 10 and 50 disposed at the upper and lower end of the core 1. One end of each tube 6 constituting the upstream-side tube group P1 is connected to the inlet-side tank 11, while the other end to the lower header member 50. One end of each tube 7 constituting the downstream-side tube group P2 is connected to the outlet-side tank 12, while the other end to the lower header member 50. The refrigerant flowed into the inlet-side tank 11 is introduced into the outlet-side tank 12 by passing through the upstream-side tube group P1, the lower header member 50 and the downstream-side tube group P2, so that the refrigerant evaporates by exchanging heat with ambient air A. Accordingly, it improves the heat exchange performance and decreases the thickness.