A gas furnace comprising: a combustion part in which a fuel gas is burnt to generate a combustion gas; a heat exchanger having a gas flow path through which the combustion gas flows; a blower configured to blow air around the heat exchanger; and an inducer configured to discharge the combustion gas from the heat exchanger, wherein the heat exchanger comprises: at least one single path having a combustion gas inlet and that form a single gas flow path, wherein the single path comprises an inflow single path having the combustion gas inlet and at least one intermediate single path disposed between the inflow single path and the multiple path; a single-single return bend disposed between the inflow single path and the intermediate single path to convert a flow direction of the combustion gas; at least one multiple path having a plurality of paths that from multiple gas flow paths, the multiple path parallel to the single path and spaced apart from the single path in one direction; and a single-multiple return bend configured to communicate with the single path and the multiple path and convert a flow direction of the combustion gas, the single-multiple return bend being disposed between the intermediate single path and the multiple path, wherein the plurality of paths are parallel to each other in the one direction, and wherein a minimum width of each of the plurality of paths decreases further away from the single path in the one direction, wherein a diameter of a gas flow path formed in the inflow single path is larger than a diameter of a gas flow path formed in the intermediate single path, and the diameter of the gas flow path formed in the intermediate single path is larger than a diameter of a gas flow path of each of the plurality of paths. a combustion part in which a fuel gas is burnt to generate a combustion gas; a heat exchanger having a gas flow path through which the combustion gas flows; a blower configured to blow air around the heat exchanger; and an inducer configured to discharge the combustion gas from the heat exchanger, wherein the heat exchanger comprises: at least one single path having a combustion gas inlet and that form a single gas flow path, wherein the single path comprises an inflow single path having the combustion gas inlet and at least one intermediate single path disposed between the inflow single path and the multiple path; a single-single return bend disposed between the inflow single path and the intermediate single path to convert a flow direction of the combustion gas; at least one multiple path having a plurality of paths that from multiple gas flow paths, the multiple path parallel to the single path and spaced apart from the single path in one direction; and a single-multiple return bend configured to communicate with the single path and the multiple path and convert a flow direction of the combustion gas, the single-multiple return bend being disposed between the intermediate single path and the multiple path, wherein the plurality of paths are parallel to each other in the one direction, and wherein a minimum width of each of the plurality of paths decreases further away from the single path in the one direction, wherein a diameter of a gas flow path formed in the inflow single path is larger than a diameter of a gas flow path formed in the intermediate single path, and the diameter of the gas flow path formed in the intermediate single path is larger than a diameter of a gas flow path of each of the plurality of paths. at least one single path having a combustion gas inlet and that form a single gas flow path, wherein the single path comprises an inflow single path having the combustion gas inlet and at least one intermediate single path disposed between the inflow single path and the multiple path; a single-single return bend disposed between the inflow single path and the intermediate single path to convert a flow direction of the combustion gas; at least one multiple path having a plurality of paths that from multiple gas flow paths, the multiple path parallel to the single path and spaced apart from the single path in one direction; and a single-multiple return bend configured to communicate with the single path and the multiple path and convert a flow direction of the combustion gas, the single-multiple return bend being disposed between the intermediate single path and the multiple path, wherein the plurality of paths are parallel to each other in the one direction, and wherein a minimum width of each of the plurality of paths decreases further away from the single path in the one direction, wherein a diameter of a gas flow path formed in the inflow single path is larger than a diameter of a gas flow path formed in the intermediate single path, and the diameter of the gas flow path formed in the intermediate single path is larger than a diameter of a gas flow path of each of the plurality of paths.