An exhaust gas purifying system is provided to, during automatic regeneration with a vehicle running, control post injection accurately through PID control even when acceleration and deceleration are repeated or when an exhaust brake valve is closed. An exhaust pipe 20 for a diesel engine 10 is connected to a DPD 25. To automatically regenerate the DPD 25, an exhaust gas temperature is detected, a deviation of the detected exhaust gas temperature from a target regeneration temperature is evaluated, and an amount of post injection is controlled through PID control according to the deviation. When, during the automatic regeneration with a vehicle running, an exhaust brake valve 24 is closed, the post injection is interrupted. While the exhaust brake is being closed, an operation of an integral control term is continued with the PID control, and when the exhaust brake valve 24 is opened, the integral control term operated without interruption is used as an initial amount of operation.