A heat exchanger having light weight and an excellent recycling property and capable of exhibiting high sealing property of an air flow passage without using an adhesive agent. This heat exchanger is configured by alternately laminating heat transfer plates A and heat transfer plates B which are respectively integrated by vacuum molding a polystyrene sheet into heat transfer surface, air flow passage rib, air flow passage end surface, groove A, protrusion, outer peripheral rib A, outer peripheral rib B, air flow passage end surface cover, and a groove B. The groove A is brought into close contact with the groove B, upper surfaces of the outer peripheral rib A and the outer peripheral rib B are brought into close contact with the heat transfer plate, the protrusion is brought into close contact with the outer peripheral rib B and the groove B, the air flow passage end surface is brought into contact with the outer peripheral rib B, the air flow passage end surface cover is brought into contact with the end surfaces of the outer peripheral rib A and the outer peripheral rib B, and the side surfaces of the outer peripheral rib A are brought into contact with each other.