To provide a post processing system of engine exhaust which improves the energy efficiency at performing freezing prevention of a reducing agent and promotion of thawing while considering the temperature distribution of the reducer, and improves the energy efficiency when self-regeneration is promoted by considering the exhaust gas temperature distribution.

A controller 50 senses the reducer temperature distribution in a storage tank 12 on the basis of a level sensor 14 and a temperature sensor 15, calculates a volume V1 of the frozen reducer on the basis of the reducer temperature distribution, calculates the mean temperature t1 of the freezing range of the reducer, and calculates a thermal energy quantity Q1 to thaw the frozen reducer. In the same manner, the exhaust gas temperature distribution is sensed on the basis of a temperature sensor 25, and the thermal energy quantity Q2 required to promote the self-regeneration is calculated. Further from a generator device 31, the electric energy quantities Q1 and Q2 are fed to electric heaters 16 and 26, respectively.