PURPOSE: To accurately adjust the height of a fluidized bed while simplifying equipment by making up for a deviation between the actual value of the height of the fluidized bed as detected based on a difference in temperature between the fluidized bed and a space above it from a set value by a discharging motion of a discharger.

CONSTITUTION: The temperature of a fluidized bed 30 is set higher by about 60-120°C than that of a thermal decomposition gas in a gas space 34. This temperature difference is utilized to detect and control the height (h) of the fluidized bed. Maintaining the specified height of the fluidized bed is very important for economical operation of a pyrolytic reactor. A thermocouple arranged in a sheath 40 detects a temperature difference between the fluidized bed 30 and the gas space 34 so that the rotational speed of a heater 194 and a carry-out speed of a conveyor screw 18 is adjusted to take a balance between the supply of wastes with the discharge of a thermal decomposition residue. This enables not only accurate adjustment of the height of the fluidized bed but also total simplification of equipment.