To provide a heat exchanger that can control temperatures of fluids to be heat-exchanged. A heat exchanger 30 includes a honeycomb structure 1, a second fluid flow through portion 26 located at an outer periphery side of the honeycomb structure 1 serving as a flow passage for a second fluid, and a third fluid flow through portion 27 located at an outer periphery side of the second fluid flow through portion 26 serving as a flow passage for a third fluid. Cells 3 of the honeycomb structure 1 serve as a first fluid flow through portion 25 through which the first fluid passes. With the heat exchanger 30, the first fluid, the second fluid, and the third fluid can be heat-exchanged without being mixed with one another. Constituting the fluid flow passage with three flow passages allows controlling temperatures of two fluids to be heat-exchanged by the residual fluid.