PURPOSE: To permit to keep the outlet temperature of gas to be heated in a high temperature for a long period of time and reduce the fluctuation thereof by a method wherein a heat accumulating body at the inflow side of heating gas is formed by particles having a heat capacity large than the same of particles constituting another heat accumulating body at the outflow side of the gas.

CONSTITUTION: Heat exchanging amount per unit volume between pebbles and gas is increased in such a condition that the smaller in the diameter of the pebbles the better, and the pebbles are provided with a property capable of being heated and cooled easily. When the pebbles having larger diameter are employed in an area I-1 near the end A and the same having smaller diameter are employed in the area I-2, the temperature in the area I-2 upon starting the supply of gas to be heated becomes high since the layer of heat accumulating body in the area I-2 is easily heated, therefore, the gas to be heated, which is provided with a high outlet temperature, may be supplied. The heat exchanging amount of the heat accumulating body layer in the area I-1 is smaller as compared with the same of the heat accumulating body layer in the area I-2, therefore, the width of temperature decrease in the area I-1 including the end portion A upon supplying the gas to be heated becomes small and the width of temperature decrease in the outlet temperature of the gas to heated becomes also small. Owing to these two effects, the outlet temperature of the gas to be heated may be kept in a high temperature and the fluctuation thereof may be reduced.