EXHAUST GAS CLEANING SYSTEM FOR INTERNAL COMBUSTION ENGINE AND EXHAUST GAS CLEANING METHOD FOR INTERNAL COMBUSTION ENGINE

An exhaust gas cleaning system for an internal combustion engine is provided with, in an exhaust passage (15) of an internal combustion engine (10) and in order from an upstream side: a hydrocarbon supply unit (32); an upstream side catalyst unit (31 a) of an oxidation catalyst, a NOx reduction catalyst, or the like; and a catalyst-equipped fine particle collection filter (31b) having a hydrocarbon adsorption function, wherein when a vehicle on which the engine (10) is mounted accelerates after driving for a preset set time period (ta) at a temperature (Tg1) of exhaust gas (G) equal to or less than a preset set exhaust gas temperature (TO), the supply of hydrocarbon into the exhaust gas (G) is started when an exhaust gas temperature (Tg) becomes equal to or more than a preset first set temperature (T1), and the supply of hydrocarbon is terminated when a preset termination condition is satisfied. As a result, when the vehicle is accelerated from steady driving in a low-speed, low-load operation state in which the temperature (Tg1) of the exhaust gas (G) is low, and a NOx generation amount is temporarily increased, the amount of NOx emitted from the internal combustion engine (10) is reduced.