A heat exchanger is applied well to a turbo-generator system and a fuel-reforming system. The heat exchanger has porous metals disposed in a hotter area and a colder area, one to each area, and a wall separating the two areas from one another. The porous metals are merged integrally with the wall through junction layers to raise the efficiency of the heat exchanger. The porous metals in the hotter and colder areas are merged together with the opposite surfaces of the wall through junction layers buried into the porous metals. The junction layers are made of pasty joining material kneaded with a powdery metal. The junction layers over the porous metals are brought into close contact with the opposite surfaces of the wall and subjected to sintering to get the porous metals merging together with the wall.