To fully exhibit the capacity of an evaporator for an ejector cycle.

The total refrigerant passage cross-sectional area on the refrigerant inlet 32 side out of a refrigerant passage 31a is made smaller than that on the refrigerant outlet 33 side. A sharp decrease in flow velocity of a refrigerant can thereby be prevented, so that a considerable lowering of a heat transfer rate of the refrigerant and a tube can be prevented to fully exhibit the capacity of the evaporator 30. Furthermore, since the decrease of flow velocity of the refrigerant can be prevented, the refrigerator oil is prevented from sticking to the internal wall of the tube 31, and in its turn, a sufficient quantity of refrigerator oil can be returned to a compressor. The lowering of the heat transfer rate caused by the refrigerator oil sticking to the internal wall can thereby be prevented while preventing the occurrence of defectiveness such as seizure of the compressor.