PURPOSE: To reduce combustion noise and stabilize the combustion by shaping a radial opening for a pilot gas hole and a pilot air hole and extending the length of a mixing passage satisfactorily.

CONSTITUTION: A pilot gas hole 1a and a pilot air hole are bored on a combustion recess 22 installed to a combustion chamber 20. These holes are radially opened on the combustion recess 22. Then, a main mixed gas hole 9a is bored on the combustion chamber 20 and it is installed to the outer peripheral part of the pilot combustion recess 22. The main mixing gas hole 9a is connected to a sufficiently long mixing passage, which forces pilot flames to extend to the outer periphery part and provides a satisfactory flame holding effect, thereby stabilizing combustion. Since a mixing passage provides a satisfactory long length required for combustion, the air and gas are mixed satisfactorily, thereby reducing combustion noise.