PURPOSE: To transmit a quantity of heat which a fuel has without waste, and save the fuel, and prevent an unfavorable phenomenon from generating by mixing in a fuel in drift air which makes a heat carrier flow, by an air diffusing plate, etc.

CONSTITUTION: A liquid fuel is refined by an air-feeding means 10 for compressed air, which is prepared separately from an air feeding means 4 for drift air, and the liquid fuel is mixed with the drift air right before the drift air enters an air diffusing pipe 3 in a furnace 1 through a valve 11 under that state. Also, a passage of the drift air is connected with a liquid fuel container with fine tubes, and the liquid fuel which rises by capillary tube phenomenon, etc., from the fine tubes is atomized by an ejecting method by the circulation of the drift air, and mixed in with the drift air. Also, a mixing chamber is formed on the bottom of the furnace 1, and the drift air by the air feeding means 4 can be fed in from the bottom of the furnace 1 using an air diffusing pipe, etc. In this case, a gas fuel (gas), separating from the drift air, is fed into the mixing chamber through a valve.