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 in which a single gas flow path is formed; a single-multiple return bend configured to communicate with the single path and convert a flow direction of the combustion gas; and at least one multiple path having a plurality of paths that form multiple gas flow paths, wherein each of the plurality of paths is configured to communicate with the single-multiple return bend in parallel with other paths in an up-downward direction, and a length of each of the plurality of paths increases as it progresses toward a lower side of the multiple path, wherein the single path comprises an inflow single path in which a combustion gas inlet is formed; and at least one intermediate single path disposed between the inflow single path and the multiple path, characterized in that the heat exchanger further comprises 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, wherein the single-multiple return bend is disposed between the intermediate single path and the multiple path. 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 in which a single gas flow path is formed; a single-multiple return bend configured to communicate with the single path and convert a flow direction of the combustion gas; and at least one multiple path having a plurality of paths that form multiple gas flow paths, wherein each of the plurality of paths is configured to communicate with the single-multiple return bend in parallel with other paths in an up-downward direction, and a length of each of the plurality of paths increases as it progresses toward a lower side of the multiple path, wherein the single path comprises an inflow single path in which a combustion gas inlet is formed; and at least one intermediate single path disposed between the inflow single path and the multiple path, characterized in that the heat exchanger further comprises 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, wherein the single-multiple return bend is disposed between the intermediate single path and the multiple path. at least one single path in which a single gas flow path is formed; a single-multiple return bend configured to communicate with the single path and convert a flow direction of the combustion gas; and at least one multiple path having a plurality of paths that form multiple gas flow paths, wherein each of the plurality of paths is configured to communicate with the single-multiple return bend in parallel with other paths in an up-downward direction, and a length of each of the plurality of paths increases as it progresses toward a lower side of the multiple path, wherein the single path comprises an inflow single path in which a combustion gas inlet is formed; and at least one intermediate single path disposed between the inflow single path and the multiple path, characterized in that the heat exchanger further comprises 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, wherein the single-multiple return bend is disposed between the intermediate single path and the multiple path.