PURPOSE: To stably burn thin gas fuel in an apparatus of simple structure, by providing clusters of pore and combustion chambers alternately in a combustion casing to let them repeat pre-heating and mixing for thin gas fuel.

CONSTITUTION: The secondary passage 2 is pnetratingly formed in the center part of a combustion casing 1, and a plurality of the primary passage 4 are also penetratingly formed along the secondary passage 2, separated by walls 3 of combustion casing. To the passages 2 and 4, three pieces of cluster of pore 5 and 7 are respectively provided, and combustion chambers 6 and 8 are provided between two each clusters of pore, 5 and 7. The other end of the primary passage 4 is inter-connected to an inteake 10a to take fuel in, as well as one ends of each passage 2 and 4 are interconnected to each other through a passage 9a, and the other end of the secondary passage 2 is interconnected to an outlet 11a to take gas out. The fuel fed from the intake 10a flows downwards, pre-heated in the clusters of pore 5 and 7 in the passages 2 and 4, and mixed in the combustion chambers 6 and 8, respectivelyay and repeatedly. With such arrangements, thin gas fuel of various density can be burnt.