PURPOSE: To improve combustion efficiency through the improvement of mixture of suction air and fuel gas.

CONSTITUTION: A rectangular opening 135 and a flow dividing plate 136 extending from one side of the opening 135 toward a place situated upper stream from a flow A of suction air and a flow B of a fuel gas are molded to the helically molded guide wall 132 of a mixer 130. A part of the flow A of suction air and the flow B of fuel gas is guided by the flow dividing plate 136 and branched to the opposite side of the guide wall 132 through the opening 135 and mixed at the interior of the mixer 130. Thereafter, the flow A of suction air and the flow B of fuel gas are guided by the helically molded guide wall 132 and flows as a swirl flow to a combustion chamber 11, whereby the flows are further completely mixed together and combustion efficiency is improved.