PURPOSE: To obtain preferable combustion characteristic of a burning device in all variable combustion quantity area by standing an intermediate cylinder having many air injection holes at the center of a combustion chamber and varying the number of opened holes in response to the alteration of fuel supply quantity.

CONSTITUTION: Nozzles 1, 1' in which fuel is fed by an electromagnetic pump 2 are provided for high and low combustions, and solenoid valves 4, 4' are respectively interposed at fuel feed pipes 3, 3' respectively connected to the nozzles 1, 1'. An intermediate cylinder 11 having many air injection holes 16 at the outer periphery, in which combustion air is supplied by a blower including a fan 13 is stood at the center on the bottom of a combustion chamber 10, and an inner cylinder 17 for elevationally dividing the holes 16 at the cylinder 11 is stood on the inner surface of the cylinder 11. A port 18 communicating with a duct 15 under the cylinders 11, 17 is opened or closed by a damper 20 which is operated by a solenoid 19 electrically interlocked with the combustion quantity switching of solenoid valves 4, 4'.