A heat exchanger includes a peripheral wall having a polygonal tube shape and partition walls that divide an inside of the peripheral wall into first cells and second cells, the first cells and the second cells extending in an axial direction of the peripheral wall. Ends of each of the first cells in the axial direction are sealed and adjacent ones of the first cells are in communication with one another so that the first cells constitute a first passage having a U-shaped cross section perpendicular to the axial direction. The first passage includes an inflow port and an outflow port that are open in the same surface of the peripheral wall. Each of the second cells constitutes a second passage including an inflow port and an outflow port provided respectively at ends of each of the second cells in the axial direction.