Emission control system and method for internal combustion engine

An emission control system for an internal combustion engine (1) is provided with a catalyst (4) disposed in an exhaust passage (3) of the internal combustion engine (1) for purifying an exhaust gas that enters the catalyst (4); engine operating state detecting means (15) for detecting an engine operating state of the internal combustion engine (1); catalyst temperature raising means capable of raising a temperature of the catalyst (4) in a plurality of modes; and selecting means (11) for selecting one mode from the plurality of modes based on a value detected by the engine operating state detecting means such that the catalyst temperature is raised. When the internal combustion engine is in a low load state, the catalyst temperature raising means raises the temperature of the catalyst in a first mode selected from the plurality of modes, by increasing an amount of a main fuel injected into the internal combustion engine (1) so as to make an air/fuel ratio closer to a stoichiometric value, and by adding a fuel into the exhaust passage (3). When the internal combustion engine is in an intermediate load state, the catalyst temperature raising means raises the temperature of the catalyst in a second mode selected from the plurality of modes, by injecting an additional fuel after injecting the main fuel into the internal combustion engine (1), and by adding the fuel into the exhaust passage (3). When the internal combustion engine is in a high load state, the catalyst temperature raising means raises the catalyst temperature in a third mode selected from the plurality of modes, by adding the fuel into the exhaust passage (3).