An exhaust gas purification system for an internal combustion engine, comprising: a pre-catalyst which is provided for an exhaust gas passage of the internal combustion engine and which has an oxidizing function; a fuel supply device which supplies fuel to the pre-catalyst; a filter which is provided for the exhaust gas passage on a downstream side from the pre-catalyst and which traps particulate matter contained in an exhaust gas, the filter including a selective catalytic reduction NOx catalyst carried thereon for reducing NOx contained in the exhaust gas by using ammonia as a reducing agent; an ammonia supply device configured to supply ammonia or an ammonia precursor to the filter; a post-catalyst which is provided for the exhaust gas passage on a downstream side from the filter, which has an oxidizing function, and which has such a function that production of N 2 based on oxidization of ammonia is facilitated in a predetermined first temperature area; and a filter regeneration process execution unit comprising means configured to execute a filter regeneration process for raising a temperature of the filter to a predetermined filter regeneration temperature at which oxidation of the particulate matter is facilitated, by supplying the fuel from the fuel supply device to the pre-catalyst so that the particulate matter accumulated on the filter is thereby oxidized and removed, characterized in that: the filter regeneration process execution unit comprises means configured to control a temperature of the exhaust gas flowing into the filter, when the filter regeneration process execution unit executes the filter regeneration process, so that,during a certain period of time, the temperature of the filter is adjusted to be in a predetermined second temperature area which is lower than the filter regeneration temperature and which provides an increasing amount of ammonia released from the carried selective catalytic reduction NOx catalyst and flowing out from the filter without being oxidized, a temperature of the post-catalyst is adjusted to be in the first temperature area, and then adjusts the temperature of the filter to the filter regeneration temperature. a pre-catalyst which is provided for an exhaust gas passage of the internal combustion engine and which has an oxidizing function; a fuel supply device which supplies fuel to the pre-catalyst; a filter which is provided for the exhaust gas passage on a downstream side from the pre-catalyst and which traps particulate matter contained in an exhaust gas, the filter including a selective catalytic reduction NOx catalyst carried thereon for reducing NOx contained in the exhaust gas by using ammonia as a reducing agent; an ammonia supply device configured to supply ammonia or an ammonia precursor to the filter; a post-catalyst which is provided for the exhaust gas passage on a downstream side from the filter, which has an oxidizing function, and which has such a function that production of N 2 based on oxidization of ammonia is facilitated in a predetermined first temperature area; and a filter regeneration process execution unit comprising means configured to execute a filter regeneration process for raising a temperature of the filter to a predetermined filter regeneration temperature at which oxidation of the particulate matter is facilitated, by supplying the fuel from the fuel supply device to the pre-catalyst so that the particulate matter accumulated on the filter is thereby oxidized and removed, characterized in that: the filter regeneration process execution unit comprises means configured to control a temperature of the exhaust gas flowing into the filter, when the filter regeneration process execution unit executes the filter regeneration process, so that,during a certain period of time, the temperature of the filter is adjusted to be in a predetermined second temperature area which is lower than the filter regeneration temperature and which provides an increasing amount of ammonia released from the carried selective catalytic reduction NOx catalyst and flowing out from the filter without being oxidized, a temperature of the post-catalyst is adjusted to be in the first temperature area, and then adjusts the temperature of the filter to the filter regeneration temperature. the filter regeneration process execution unit comprises means configured to control a temperature of the exhaust gas flowing into the filter, when the filter regeneration process execution unit executes the filter regeneration process, so that,during a certain period of time, the temperature of the filter is adjusted to be in a predetermined second temperature area which is lower than the filter regeneration temperature and which provides an increasing amount of ammonia released from the carried selective catalytic reduction NOx catalyst and flowing out from the filter without being oxidized, a temperature of the post-catalyst is adjusted to be in the first temperature area, and then adjusts the temperature of the filter to the filter regeneration temperature.