An exhaust gas purification system for an internal combustion engine provided with an engine, an exhaust passage where exhaust gas exhausted from the engine passes, and an exhaust gas purifier installed in the exhaust passage, the system comprising: an air flow control unit for reducing a flow rate of air to be supplied to the engine so as to raise a temperature of the exhaust gas exhausted from the engine; and an activation timing control unit for controlling a timing of activating the air flow control unit; wherein the activation timing control unit is configured to control the timing of activating the air flow control unit so that a combustion state in the engine does not become unstable when the air flow control unit is activated and the air to be supplied to the engine is reduced; wherein the activation timing control unit is configured to estimate a temperature and a pressure in a cylinder of the engine and control the timing of activating the air flow control unit based on the estimated temperature and pressure in the cylinder of the engine; and wherein the activation timing control unit is configured to: estimate a temperature and a pressure in a cylinder at a fuel injection timing of the engine; determine whether or not an inside of the cylinder after the air flow control is in a stable combustion state based on a stable combustion determination map having the estimated temperature and pressure in the cylinder at the fuel injection timing of the engine as input data; and when it is determined that the inside of the cylinder after the air flow control is in the stable combustion state, then activate the air flow control unit. an air flow control unit for reducing a flow rate of air to be supplied to the engine so as to raise a temperature of the exhaust gas exhausted from the engine; and an activation timing control unit for controlling a timing of activating the air flow control unit; wherein the activation timing control unit is configured to control the timing of activating the air flow control unit so that a combustion state in the engine does not become unstable when the air flow control unit is activated and the air to be supplied to the engine is reduced; wherein the activation timing control unit is configured to estimate a temperature and a pressure in a cylinder of the engine and control the timing of activating the air flow control unit based on the estimated temperature and pressure in the cylinder of the engine; and wherein the activation timing control unit is configured to: estimate a temperature and a pressure in a cylinder at a fuel injection timing of the engine; determine whether or not an inside of the cylinder after the air flow control is in a stable combustion state based on a stable combustion determination map having the estimated temperature and pressure in the cylinder at the fuel injection timing of the engine as input data; and when it is determined that the inside of the cylinder after the air flow control is in the stable combustion state, then activate the air flow control unit. estimate a temperature and a pressure in a cylinder at a fuel injection timing of the engine; determine whether or not an inside of the cylinder after the air flow control is in a stable combustion state based on a stable combustion determination map having the estimated temperature and pressure in the cylinder at the fuel injection timing of the engine as input data; and when it is determined that the inside of the cylinder after the air flow control is in the stable combustion state, then activate the air flow control unit.