provided at a proper position between an intake port 551 and a nozzle 552 both being described later and a numeral 400 generally designates a pilot burner means for heating the heating medium 200 a conduit 500 which constitutes a part of a feeding means for feeding gas for combustion such as air is provided in the combustion chamber 110 so as to direct the nozzle 552 formed at an end of the conduit upwardly the intake port 551 is formed in the bottom wall 112 connect the combustion chamber with the conduit 500 extending laterally below the bottom wall 112 at a junction therebetween however it is possible that the conduit 500 is introduced in the combustion chamber through the side circumferential wall 111 of the combustion chamber and the intake port is formed at a part of the conduit extending laterally in the combustion chamber so that the opening of the intake port is directed upwardly in the combustion chamber having the construction as above mentioned when gas for combustion such as air is forcibly fed through the conduit 500 a part of the heating medium is sucked through the intake port 551 and is discharged from the nozzle 552 together with air in this case fuel such as a type heavy oil kerosine is fired by electric discharge in the pilot burner 440 the heating medium discharged from the nozzle 552 of the conduit is heated by flames 441 from the pilot burner or a hot gas produced by the pilot burner air is supplied through the pilot burner means 400 into the combustion chamber to spread.