A heat exchanger includes a bag-like outer packaging material. A heat medium flows into an inside of the outer packaging material. An inner core material is arranged in the inside of the outer packaging material. The outer packaging material has an outer packaging laminate material including a metal heat transfer layer and a resin thermal fusion layer on a surface side of the heat transfer layer. The outer packaging laminate materials form a bag shape by integrally joining the thermal fusion layers along the peripheral edge portions. The inner core material includes the inner core laminate material with a metal heat transfer layer and resin thermal fusion layers on surface sides of the heat transfer layer. The thermal fusion layers of a concave portion bottom and a convex portion top of the inner core material and the thermal fusion layers of the outer packaging laminate material are integrally joined.