An exhaust system comprising: an exhaust pipe through which an exhaust gas flows, the exhaust gas being generated by an engine having a first injector injecting a fuel into a combustion chamber; a second injector mounted at the exhaust pipe and injecting a reducing agent; a DeNOx catalyst mounted at the exhaust pipe downstream of the second injector and reducing NOx contained in the exhaust gas by using the reducing agent injected by the second injector; and a control unit controlling the second injector so as to regenerate the DeNOx catalyst and calculating mass of NO2 and mass of the NOx remaining in the DeNOx catalyst after regeneration, wherein the control unit calculates total mass flow of the reducing agent used in the regeneration of the DeNOx catalyst, calculates each mass flow of the reducing agent used respectively in nitrate reduction reaction, NO2 reduction reaction, and simple oxidation reaction of the reducing agent, and calculates mass flow of the NO2 released from the DeNOx catalyst and mass flow of reduced NO2 by using each mass flow of the reducing agent, wherein the control unit calculates the total mass flow of the reducing agent from utilization efficiency of the reducing agent and mass flow of the injected reducing agent, wherein the control unit calculates the mass flow of the reducing agent used in the nitrate reduction reaction from the temperature of the DeNOx catalyst, a lambda of an inlet of the DeNOx catalyst, the mass of the NOx stored in the DeNOx catalyst, and the total mass flow of the reducing agent, wherein the control unit calculates the mass flow of the reducing agent used in the NO2 reduction reaction from the mass of the NO2 stored in the DeNOx catalyst, the temperature of the DeNOx catalyst, and the total mass flow of the reducing agent, and wherein the control unit calculates the mass flow of the reducing agent used in the simple oxidation reaction of the reducing agent from the temperature of the DeNOx catalyst, a lambda of an engine outlet, and the total mass flow of the reducing agent. an exhaust pipe through which an exhaust gas flows, the exhaust gas being generated by an engine having a first injector injecting a fuel into a combustion chamber; a second injector mounted at the exhaust pipe and injecting a reducing agent; a DeNOx catalyst mounted at the exhaust pipe downstream of the second injector and reducing NOx contained in the exhaust gas by using the reducing agent injected by the second injector; and a control unit controlling the second injector so as to regenerate the DeNOx catalyst and calculating mass of NO2 and mass of the NOx remaining in the DeNOx catalyst after regeneration, wherein the control unit calculates total mass flow of the reducing agent used in the regeneration of the DeNOx catalyst, calculates each mass flow of the reducing agent used respectively in nitrate reduction reaction, NO2 reduction reaction, and simple oxidation reaction of the reducing agent, and calculates mass flow of the NO2 released from the DeNOx catalyst and mass flow of reduced NO2 by using each mass flow of the reducing agent, wherein the control unit calculates the total mass flow of the reducing agent from utilization efficiency of the reducing agent and mass flow of the injected reducing agent, wherein the control unit calculates the mass flow of the reducing agent used in the nitrate reduction reaction from the temperature of the DeNOx catalyst, a lambda of an inlet of the DeNOx catalyst, the mass of the NOx stored in the DeNOx catalyst, and the total mass flow of the reducing agent, wherein the control unit calculates the mass flow of the reducing agent used in the NO2 reduction reaction from the mass of the NO2 stored in the DeNOx catalyst, the temperature of the DeNOx catalyst, and the total mass flow of the reducing agent, and wherein the control unit calculates the mass flow of the reducing agent used in the simple oxidation reaction of the reducing agent from the temperature of the DeNOx catalyst, a lambda of an engine outlet, and the total mass flow of the reducing agent.