Abstract: 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 largely the efficiency of the heat exchanger. Heat is transferred from the hotter area where a fluid is allowed to flow through there to the colder area where another fluid is allowed to pass through there through the wall isolating the two fluids different in temperature from one another. 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.