Disclosed is a gas furnace including a mixer configured to mix air and fuel gas introduced from an intake pipe and a manifold respectively so as to produce an air-fuel mixture, a mixing pipe configured to allow the air-fuel mixture having passed through the mixer to flow therein, a burner assembly configured to combust the air-fuel mixture having passed through the mixing pipe so as to generate combustion gas, heat exchangers configured to allow the combustion gas to flow therein, an exhaust pipe configured to discharge exhaust gas, which is the combustion gas having passed through the heat exchangers, to the outside. The gas furnace further includes a recirculator installed around the exhaust pipe and configured to guide a portion of the exhaust gas flowing in the exhaust pipe to the mixer, and may thus greatly reduce or fundamentally block NOx emissions.