A combustor according to the present invention comprises: a combustion chamber having a grate provided therein and a combustion space formed above the grate; a fuel supply part downwardly connected to the center part of the grate so as to supply fuel to an upper part of the grate; an air supply part connected to a side part of the combustion chamber so as to be slanted on a horizontal plane, thereby supplying combustion air such that combustion air swirls in the combustion space; a guide member disposed to protrude toward the inside of the combustion space from an inlet through which the combustion air flows into the combustion space, and opened downward so as to guide the combustion air such that the combustion air, provided by the air supply part, swirls downward along an inner wall of the combustion chamber in the combustion space; and protruding guide lines having a screw line shape and extended to be slanted downward on at least one of the respective surfaces, which face each other, from the guide member and the inner wall of the combustion chamber.