A heat exchanger, includes a housing including an inlet passage through which a fluid is introduced, an outlet passage from which the fluid is discharged, an inner communication space in which the fluid flows from the inlet passage and the outlet passage, and a plurality of unit plates of thin flat plates that are disposed in parallel to a flowing direction of the fluid, the plurality of unit plates layered in a direction to be substantially orthogonal to the flowing direction, the plurality of unit plates each having a fin in parallel to the flowing direction of the fluid, a frame enclosing the fin and projecting in an out-of-plane direction of the fin, and holes respectively communicating with the inlet passage and the outlet passage. Heat is exchanged between a heating body attached on an outer surface of the housing and the fluid.