PURPOSE: To provide stable combustion even if attenuated combustion with a large flow rate is carried out by providing an auxiliary blow-out groove between the main blow-out ports from which attenuated mixture gas is blown out and blowing out concentrated gas fuel from the auxiliary blow-out grooves.

CONSTITUTION: The combustion section 40 has many main blow-out ports 41 which blow the attenuated mixture gas from an evaporation chamber 13 upwards and auxiliary blow-out grooves 42 that are constituted among the main blow-out ports. And in the auxiliary blow-out grooves 42 a combustion gas supply pipe 53 is provided with an opening 54. The gas fuel that is supplied from said opening 56 spreads as a concentrated gas fuel in the auxiliary blow- out grooves 42 and, further, is blown out upwards from the auxiliary blow-out grooves 42, and a concentrated gas fuel goes up in a ceramic plate 63 with many ports and on its upper face side flames S are formed against the main flames M, and, at the same time, part of the concentrated gas fuel mixes with the attenuated gas that is blown out from the main blow-out ports 41, and brings the rate of attenuation of the attenuated mixture gas near to the rate at which the combustion speed is maximum.