PURPOSE: To enable a soaking heating to be carried out by a method wherein a space into which or out of which a boggie having a unified rotary kiln mounted thereon is charged is arranged between a combustion assisting member and an air blower.

CONSTITUTION: Boggies 4 having a unified rotary kiln 3 mounted thereon with material to be treated stored therein are positioned at a right space 5 and a left space 5, respectively. Air is injected from one blower 2 into its corresponding rotary kiln 3. In the case that an inner area of the rotary kiln 3 is already kept at its heated state, this rotary kiln 3 is kept at its cooling mode, so that the material to be treated is cooled with air. The air is heated in advance with this cooling operation and guided into a unified rotary kiln 3' located at an opposite side through the combustion assisting member 1. During this feeding operation, since the fuel is being injected from a fuel nozzle 7 into the opposite side rotary kiln 3', the preheated air is mixed with this fuel and then combustion is carried out within the rotary kiln 3'. With such an arrangement as above, it is possible to perform a soaking heating operation.