A heat exchanger includes a plurality of fluid passages through which a heat-exchanger fluid including a liquid-phase fluid passes, a tank disposed above inlet parts of the fluid passages for distributing a flow of the heat-exchanger fluid to the fluid passages, and a retention member that is located above the inlet parts within the tank, for temporarily storing therein the liquid-phase fluid flowing into the tank. The retention member is constructed such that the liquid-phase fluid overflowing from the retention member falls toward the inlet part. Accordingly, the heat-exchanger fluid can be uniformly distributed into the fluid passages from the tank. For example, the heat exchanger can be used as an evaporator for a refrigerant cycle device having an ejector.