A power apparatus including a reducing agent supply control system includes: 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 the pressure of air which is supplied to the engine; a nitrogen oxide concentration sensor installed on the exhaust passage, and configured to measure the 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 the 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.