A power apparatus including a reducing agent supply control system, the power apparatus comprising: an engine configured to emit exhaust gas containing nitrogen oxide by burning air and fuel at a preset air-fuel ratio; an exhaust passage configured such that the exhaust gas emitted by the engine moves therethrough; a pressure sensor configured to actually measure a pressure of air which is supplied to the engine; a nitrogen oxide concentration sensor installed on the exhaust passage, and configured to measure a nitrogen oxide (NOx) concentration of the exhaust gas; a reducing agent supply unit configured to supply a reducing agent to the exhaust gas which moves along the exhaust passage; and a control unit configured to determine an amount of reducing agent to be supplied based on information received from the pressure sensor and the nitrogen oxide concentration sensor, and to control the reducing agent supply unit, characterized in that the control unit is configured to: calculate a mass of air for each stroke of one or more cylinders of the engine from the pressure of air, actually measured by the pressure sensor, by using preset correlation data between a pressure and mass of air to be supplied to the engine; calculate a mass of air per hour by using a rotation speed of the engine, a total number of cylinders of the engine, and the calculated mass of air for each stroke; calculate a mass of fuel per hour by dividing the mass of air per hour by an air-fuel ratio; and calculate a total amount of nitrogen oxide emitted from the engine by using the nitrogen oxide (NOx) concentration measured by the nitrogen oxide concentration sensor, the mass of air per hour, and the mass of fuel per hour, and determines an amount of reducing agent to be supplied, which reduces the total amount of nitrogen oxide. an engine configured to emit exhaust gas containing nitrogen oxide by burning air and fuel at a preset air-fuel ratio; an exhaust passage configured such that the exhaust gas emitted by the engine moves therethrough; a pressure sensor configured to actually measure a pressure of air which is supplied to the engine; a nitrogen oxide concentration sensor installed on the exhaust passage, and configured to measure a nitrogen oxide (NOx) concentration of the exhaust gas; a reducing agent supply unit configured to supply a reducing agent to the exhaust gas which moves along the exhaust passage; and a control unit configured to determine an amount of reducing agent to be supplied based on information received from the pressure sensor and the nitrogen oxide concentration sensor, and to control the reducing agent supply unit, calculate a mass of air for each stroke of one or more cylinders of the engine from the pressure of air, actually measured by the pressure sensor, by using preset correlation data between a pressure and mass of air to be supplied to the engine; calculate a mass of air per hour by using a rotation speed of the engine, a total number of cylinders of the engine, and the calculated mass of air for each stroke; calculate a mass of fuel per hour by dividing the mass of air per hour by an air-fuel ratio; and calculate a total amount of nitrogen oxide emitted from the engine by using the nitrogen oxide (NOx) concentration measured by the nitrogen oxide concentration sensor, the mass of air per hour, and the mass of fuel per hour, and determines an amount of reducing agent to be supplied, which reduces the total amount of nitrogen oxide.