1. An exhaust gas post processing method, comprising: regenerating a diesel particulate filter; reducing a remainder in the diesel particulate filter by raising the concentration of a reducing agent included in the exhaust gas after the regeneration; oxidizing a remainder in the diesel particulate filter by raising the concentration of oxygen included in the exhaust gas after the regeneration; and calculating a real ash amount trapped in the diesel particulate filter by detecting a front/rear pressure difference of the diesel particulate filter after the reducing and the oxidation.