To avoid uneven expansion of ceramic straight tubes and to make them durable even in the use in a high-temperature region by forming a combustion gas burning chamber and a combustion product exhaust chamber isolated from each other in a furnace and by making the chambers communicate loosely with each other by the ceramic straight tubes.

A combustion gas burning chamber 11 is connected to a burner through an opening part of the top wall of a furnace. A combustion product exhaust chamber 13 is connected to an exhaust device with a heat exchanger through another opening part of the top wall of the furnace and via an exhaust tube. Since fuel can be burnt completely in the combustion gas burning chamber 11, a combustion flame having a high temperature locally does not enter ceramic straight tubes 20 and only exhaust gas after combustion passes therethrough. Therefore it is possible to avoid uneven expansion of the straight tubes 20 due to the local high temperature and to prolong the lifetime of the straight tubes.