Exhaust gas purification system of internal combustion engine and exhaust gas purification method of internal combustion engine

An exhaust gas post-treatment device (31) provided with an oxidation catalyst (31a) and a DPF (31b) in the stated order from an upstream side is provided to an exhaust passage (15) of an internal combustion engine (10). After the operating state of the internal combustion engine (10) has transitioned from a travel operating state to an idling operating state, a catalyst indicative temperature (T) indicating the temperature of the oxidation catalyst (31a) is in a set temperature range (Rab) between a lower-limit set temperature (Ta) and an upper-limit set temperature (Tb) which have been set in advance, and when an estimated PM accumulated amount (V) of the DPF (31b) is less than a regeneration starting threshold (Vc) set in advance, a target value (Et) of an EGR rate (E) of EGR control is set to a second EGR rate (E2) higher than a first EGR rate (E1) during normal operation of the internal combustion engine (10), and EGR control is performed. In the idling operating state after the vehicle has traveled, the production of NO2 by the oxidation catalyst (31a) provided to the exhaust gas post-treatment device (31) of the exhaust passage (15) is thereby reduced to suppress the discharge of NO2 into the atmosphere.