EXHAUST-GAS PURIFICATION SYSTEM

Provided is an exhaust gas purification system in which exhaust pipe injection by PID control can be accurately controlled in traveling automatic regeneration even with repeated acceleration and deceleration or even when an exhaust brake valve is closed. A DPD 25 is connected to an exhaust pipe 20 of a diesel engine 10, an exhaust gas temperature during automatic regeneration of the DPD 25 is detected, and the difference between the detected exhaust gas temperature and a target regeneration temperature is determined. In a case where an exhaust pipe injection amount is PID controlled based on this difference, when an exhaust brake valve 24 is closed during traveling automatic regeneration, the exhaust pipe injection is stopped and calculation of an integral control term by the PID control is continued as long as the exhaust valve brake valve 24 is closed, and when the exhaust brake valve 24 is opened, the integral control term that has been continuously calculated is taken as an initial operation amount.