PURPOSE: To prolong effective steam generating time in a furnace and to attempt reduction in combustion residual ash of organic waste, by operating two carbonization furnaces, which are provided for one heating apparatus of the waste by means of combustion of carbonization gas, under a time difference operating condition.

CONSTITUTION: Carbonization gas is produced by feeding organic waste in a carbonization furnace and a heating apparatus is heated by burning the carbonization gas produced. At that time, two carbonization furnaces are operated for one heating apparatus in combination. Moreover, two carbonization furnaces are operated under a time difference condition, and the carbonization gas is alternating produced by overlapping the carbonization initiating time of the first carbonization furnace on the carbonization terminating time of the second carbonization furnace, and gas thus produced is burnt.