A combustor has at least one premixing burner for premixing fuel with air and jetting the mixed gas into a chamber for combustion. A cylinder attached to an outer circumferential portion of an end face of the burner is provided with air supply holes. An interval D1 defined between the adjacent air supply holes and an interval D2 defined between each air supply hole and the end face of the burner are each made narrower than the quenching distance in the premixed gas jetted from the premixing burner.