A heat exchanger includes a heat exchanger core section, a refrigerant inlet header section, a refrigerant outlet header section, a refrigerant inflow intermediate header section and a refrigerant outflow intermediate header section. A flow-dividing control wall having a plurality of refrigerant passage holes and vertically dividing the interior of the second intermediate header section into two spaces is provided within the second intermediate header section. A communication is established between the first intermediate header section and the lower space of the second intermediate header section at one end. A guide portion is provided at a refrigerant inflow end portion of the second intermediate header section so as to guide the refrigerant flowing into the lower space of the second intermediate header section such that the refrigerant flows toward the flow-dividing control wall; i.e., toward the upper side.