A combustor and a combustion method for the combustor, which can suppress backfire and ensure stable combustion. The combustor comprises a mixing-chamber forming member for forming therein a mixing chamber in which air for combustion and fuel are mixed with each other, and a combustion chamber for burning a gas mixture mixed in the mixing chamber and producing combustion gases. A channel for supplying the air for combustion to the mixing chamber from the outer peripheral side of the mixing-chamber forming member is provided inside the mixing-chamber forming member. The fuel and the air are premixed in the channel, and a resulting premixed gas mixture is supplied to the mixing chamber.