An exhaust gas purification system comprising: a NOx storage reduction catalyst disposed in an exhaust pipe of an engine; and a desulfurization process control unit configured to control an amount of intake air introduced to the engine to bring an exhaust gas in a rich condition, and perform a desulfurization process to the NOx storage reduction catalyst, the desulfurization process control unit being configured to gradually reduce the amount of intake air when the desulfurization process starts, characterized in that the desulfurization process control unit includes: a target intake air amount calculating unit configured to obtain a target amount of intake air based on engine parameters of the engine; a coefficient map of changing rate that decides a coefficient of changing rate in accordance with deviation between the target amount of intake air and an actual amount of intake air, and time elapsing from start of the desulfurization process; a transitional target intake air amount calculating unit configured to obtain a transitional target amount of intake air by multiplying the target amount of intake air, which is obtained by the target intake air amount calculating unit, by the coefficient of changing rate, which is obtained from the coefficient map; and an intake air amount control unit configured to control the amount of intake air such that the amount of intake air introduced to the engine becomes equal to the transitional target amount of intake air, which is obtained by the transitional target intake air amount calculating unit. a NOx storage reduction catalyst disposed in an exhaust pipe of an engine; and a desulfurization process control unit configured to control an amount of intake air introduced to the engine to bring an exhaust gas in a rich condition, and perform a desulfurization process to the NOx storage reduction catalyst, the desulfurization process control unit being configured to gradually reduce the amount of intake air when the desulfurization process starts, characterized in that the desulfurization process control unit includes: a target intake air amount calculating unit configured to obtain a target amount of intake air based on engine parameters of the engine; a coefficient map of changing rate that decides a coefficient of changing rate in accordance with deviation between the target amount of intake air and an actual amount of intake air, and time elapsing from start of the desulfurization process; a transitional target intake air amount calculating unit configured to obtain a transitional target amount of intake air by multiplying the target amount of intake air, which is obtained by the target intake air amount calculating unit, by the coefficient of changing rate, which is obtained from the coefficient map; and an intake air amount control unit configured to control the amount of intake air such that the amount of intake air introduced to the engine becomes equal to the transitional target amount of intake air, which is obtained by the transitional target intake air amount calculating unit. a target intake air amount calculating unit configured to obtain a target amount of intake air based on engine parameters of the engine; a coefficient map of changing rate that decides a coefficient of changing rate in accordance with deviation between the target amount of intake air and an actual amount of intake air, and time elapsing from start of the desulfurization process; a transitional target intake air amount calculating unit configured to obtain a transitional target amount of intake air by multiplying the target amount of intake air, which is obtained by the target intake air amount calculating unit, by the coefficient of changing rate, which is obtained from the coefficient map; and an intake air amount control unit configured to control the amount of intake air such that the amount of intake air introduced to the engine becomes equal to the transitional target amount of intake air, which is obtained by the transitional target intake air amount calculating unit.