The present invention relates to a technology of reducing nitrogen oxide (NOx) which is harmful discharge gas discharged from an internal combustion engine or a combustor, and to an exhaust gas purification system which inputs solid ammonium salt such as ammonium carbamate or ammonium carbonate into a reactor, thermally decomposes and converts the solid ammonium salt into the ammonia by using engine cooling water, exhaust gas, or an electric heater, which are installed in the reactor, and reduces the nitrogen oxide included in the exhaust pipe on a selective catalytic reduction into nitrogen by injecting the ammonia by using a pressure regulator and a dosing valve.