To protect an outer tube against damage due to high temperature in a heat exchanger which can recover heat from high temperature combustion gas generated from a furnace for combusting refuse at a high temperature.

In a double tube heat exchanger having an outer tube 10 and an inner tube 14, protrusions and recesses 12, e.g. ridge-like protrusions or columnar protrusions, are formed on the inner face at the top 11 of hemispherical closed end of the outer tube in order to increase the contact area between a heat exchanging fluid pressure fed from the inner tube 14 and the inner face at the closed end of the outer tube thus cooling a required part of the outer tube 10 simultaneously with heat exchange.