An exhaust gas purification system for an internal combustion engine that includes a diesel oxide catalyst DOC and a diesel particulate filter DPF for collecting particulate matter in exhaust gas in an exhaust gas passage and that treats the PM collected in the diesel particulate filter DPF to regenerate the diesel particulate filter DPF, the system comprising: a regeneration control unit controlling an amount of late post-injection that is performed in a period after a main injection and does not directly contribute to combustion, when the PM has accumulated more than a predetermined amount, to heat up the diesel particulate filter DPF to around a predetermined target temperature and burn off the accumulated PM, the regeneration control unit including a feedforward controller (47,81) outputting a basic injection amount for the amount of the late post-injection based on an operating condition of the internal combustion engine, a feedback controller outputting a correction amount of late post-injection for achieving the target temperature of the diesel particulate filter DPF, and an injection amount adding unit adding the correction amount of late post-injection output from the feedback controller to the basic injection amount output from the feedforward controller (47, 81) to compute the amount of late post-injection, the system further comprising: at least one of an integrator resetter resetting an integral value of an integrator forming the feedback controller when a sudden drop in exhaust gas flow rate is detected based on a monitored exhaust gas flow rate or a control value calculated from the exhaust gas flow rate which has decreased to a threshold or below, and a basic injection amount calculating unit calculating the basic injection amount to be output from the feedforward controller based on the exhaust gas flow rate or a control value calculated from the exhaust gas flow rate. a regeneration control unit controlling an amount of late post-injection that is performed in a period after a main injection and does not directly contribute to combustion, when the PM has accumulated more than a predetermined amount, to heat up the diesel particulate filter DPF to around a predetermined target temperature and burn off the accumulated PM, the regeneration control unit including a feedforward controller (47,81) outputting a basic injection amount for the amount of the late post-injection based on an operating condition of the internal combustion engine, a feedback controller outputting a correction amount of late post-injection for achieving the target temperature of the diesel particulate filter DPF, and an injection amount adding unit adding the correction amount of late post-injection output from the feedback controller to the basic injection amount output from the feedforward controller (47, 81) to compute the amount of late post-injection, the system further comprising: at least one of an integrator resetter resetting an integral value of an integrator forming the feedback controller when a sudden drop in exhaust gas flow rate is detected based on a monitored exhaust gas flow rate or a control value calculated from the exhaust gas flow rate which has decreased to a threshold or below, and a basic injection amount calculating unit calculating the basic injection amount to be output from the feedforward controller based on the exhaust gas flow rate or a control value calculated from the exhaust gas flow rate.