A method for controlling an exhaust gas purification system comprising an exhaust gas purification apparatus in which exhaust gas of an internal combustion engine is purified when the exhaust gas is in a lean air-fuel ratio state, and in which purification capability is regenerated when the exhaust gas is in an rich air-fuel ratio state, the method for controlling the exhaust gas purification system performing, in a supply control of unburned fuel for making the exhaust gas into a rich air-fuel ratio state, both post-injection by which post-injection is performed as in-cylinder injection and in-exhaust-pipe direct injection by which fuel is injected directly into the exhaust pipe, the method comprising when an operating state of the internal combustion engine is in low load, the supply of the unburned fuel is controlled by using the post-injection without using the in-exhaust-pipe direct injection, when the operating state of the internal combustion engine is in medium load, the supply of the unburned fuel is controlled by using both the post-injection and the in-exhaust-pipe direct injection, when the operating state of the internal combustion engine is in high load, the supply of unburned fuel is controlled by using the in-exhaust-pipe direct injection without using the post-injection, and in the supply control of the unburned fuel in medium load, a target addition amount of unburned fuel (Qr) to be added for making the exhaust gas into a rich air-fuel ratio state is calculated by dividing an intake air amount (Va) by a target air-fuel ratio (At) calculated from the operating state of the internal combustion engine, and by then subtracting, from the divided value, a fuel flow amount required depending on the operating state of the internal combustion engine, the fuel flow amount is an amount of fuel necessary for the engine to generate a demanded output in a normal driving, a post-injection amount is calculated by calculating an addition amount ratio (Rp), which is a ratio between the post-injection amount (Qp) and an in-exhaust-pipe direct injection amount (Qe), from the operating state of the internal combustion engine, and by then multiplying the target addition amount (Qr) by the addition amount ratio (Rp), the in-exhaust-pipe direct injection amount (Qe) is calculated by subtracting the post-injection amount (Qp) from the target addition amount (Qr), the unburned fuel is supplied in the calculated post-injection amount (Qp) and the calculated in-exhaust-pipe direct injection amount (Qe), and the addition amount ratio (Rp) is set so that the amount of the in-exhaust-pipe direct injection (Qe) is increased in terms of the target addition amount (Qr) as the load of the engine increases so as to reduce the generation of white smoke and black smoke. a target addition amount of unburned fuel (Qr) to be added for making the exhaust gas into a rich air-fuel ratio state is calculated by dividing an intake air amount (Va) by a target air-fuel ratio (At) calculated from the operating state of the internal combustion engine, and by then subtracting, from the divided value, a fuel flow amount required depending on the operating state of the internal combustion engine, the fuel flow amount is an amount of fuel necessary for the engine to generate a demanded output in a normal driving, a post-injection amount is calculated by calculating an addition amount ratio (Rp), which is a ratio between the post-injection amount (Qp) and an in-exhaust-pipe direct injection amount (Qe), from the operating state of the internal combustion engine, and by then multiplying the target addition amount (Qr) by the addition amount ratio (Rp), the in-exhaust-pipe direct injection amount (Qe) is calculated by subtracting the post-injection amount (Qp) from the target addition amount (Qr), the unburned fuel is supplied in the calculated post-injection amount (Qp) and the calculated in-exhaust-pipe direct injection amount (Qe), and