PURPOSE: To reduce NOx production without using a circulating fan by a method wherein exhaust gas is introduced into one of cylindrical parts of a heat exchanger which is divided into two parts while combustion air is introduced into the other opposing cylindrical part, and speed of rotation of the heat exchanger is changed.

CONSTITUTION: A cylindrical part of a rotary heat exchanger 10 is divided in the axial direction, and a heat accumulator is fixed in the cylindrical part which is divided in two or more parts. Exhaust gas (b) is introduced into one of the divided cylindrical parts of the heat exchanger 10, while combustion air (a) is introduced into the another cylindrical part in a direction opposing to the one in which the exhaust gas (b) flows. Speed of rotation of the heat exchanger 10 is changed to 1 to 10 rpm. By this, heat exchanger occurs between the exhaust gas (b) and the combustion air (a), and 1 to 10 of the exhaust gas after the heat exchange is circulated by entrain leak onto the side of the combustion air after the heat exchange. Accordingly, O2 control becomes possible and a low NO2 combustion at a combustion burner also becomes possible.