A heat exchanger comprises a first header (1), a second header (2) spaced apart from the first header (1) by a predetermined distance, flat tubes (3) each of which defines two ends connected to the first and second headers (1, 2), respectively, to communicate the first and second headers (1, 2), fins (4) interposed between adjacent flat tubes (3), and a flow member defining a fluid-flow passage and forming a partition-wall-type heat-exchanging unit with the second header (2). A refrigeration system comprises a compressor, a condenser, a throttle device, and an evaporator that is the heat exchanger. The compressor, condenser, throttle device, and evaporator are connected sequentially, and at least part of refrigerant flowing out of an outlet of the condenser enters into the flow member to exchange heat with a refrigerant flowing out of an outlet of the evaporator.