PURPOSE: To restrain the generating amount of carbon monoxide even upon setting weak combustion, by a method wherein the diameter of the upper part of an inner flame tube, arranged concentrically with an outer flame tube and provided with a plurality of venting holes, is formed so as to be smaller than the diameter of the lower part of the same.

CONSTITUTION: When the condition of weak combustion is set, combustible mixture is generated near the upper end of a wick 62, the temperature of the wick 62 is increased, the amount of receiving heat of a fire pan 60 is increased, the amount of heat radiation through a heat absorbing pipe 114 is increased and the wick 62 is kept in a temperature substantially equal to the temperature upon strong combustion. Unburnt gas is burnt while forming a secondary flame at the slightly upper position of a boundary between an lower inner flame tube 96 and an upper inside flame tube 97 by air which flows into venting holes 90, 103 while ascending through a combustion chamber 84. The diameter of the inner flame tube 97 is made smaller at this part, therefore, the secondary flame is burnt perfectly without contacting with the main body 92 of the inner flame tube. The diameter of the upper inner flame tube 97 is made so as to be 0.75W0.85 times of the diameter of the lower inner flame tube 96 and the ratio in the height direction of the tube is made 1:3 substantially, whereby the generation of poisonous gas upon weak combustion may be restrained and red flame upon strong combustion may be eliminated.