A heat exchanger is provided with a header tank having therein a circulation portion in which fluid flows, and multiple tubes which are stacked in a longitudinal direction of the header tank. The circulation portion is communicated with interiors of the tubes, and partitioned into an inlet side passage and other passages. An inflow port member is arranged at a longitudinal-direction end of the inlet side passage, and provided with multiple openings for causing at least a mainstream flow and a substream flow of fluid introduced toward the tubes. The mainstream flow is substantially evenly flow-divided by the substream flow.