a pilot nozzle provided at a center of an axis of the combustor, and performing diffusion combustion;

main nozzles provided circumferentially around the pilot nozzle so as to be equally spaced with each other, and performing premixed combustion;

a combustor basket covering outside circumferences of the pilot nozzle and the main nozzles;

a cylinder connected to a first distal end, on first proximal sides of the main nozzles, of the combustor basket,

having an expanding outside wall which is formed from first distal sides of the main nozzles to the first proximal sides of the main nozzles, and

having a second distal end, on the first proximal sides of the main nozzles, which is formed in a semicircular cross-sectional shape;

an external cylinder provided around an outside circumference of the combustor basket and the cylinder so as to form a passageway of a compressed air between an inside wall thereof and an outside wall of the combustor basket and the expanding outside wall of the cylinder; and

notches each of which is formed at tips on the second distal end of the cylinder by providing a slot which forms a step, at a position corresponding to a radial line connecting an intermediate position of an adjacent pair of the main nozzles and a center axis of the combustor, wherein

the notches are grooves which are indented with respect to the other surfaces of the second distal end.