An exhaust gas purification system for an internal combustion engine, comprising: a selective reduction type NO x catalyst to be provided in an exhaust passage of the internal combustion engine and configured to selectively reduce NO x contained in exhaust gas when a reducing agent is supplied thereto; a supply apparatus that is configured to supply the reducing agent to the selective reduction type NO x catalyst; a controller configured to: calculate a reduction consumption amount, which is an amount of the reducing agent amount consumed per unit time by the selective reduction type NO x catalyst for reduction of the NO x; calculate a desorption amount, which is an amount of the reducing agent, from among the reducing agent adsorbed to the selective reduction type NO x catalyst, that is desorbed from the selective reduction type NO x catalyst per unit time without being consumed for reduction of the NO x, and when the reducing agent adsorption amount adsorbed to the selective reduction type NO x catalyst is held at the target adsorption amount, control a reducing agent supply amount supplied from the supply apparatus per unit time to an amount obtained by adding the calculated reduction consumption amount and desorption amount. a selective reduction type NO x catalyst to be provided in an exhaust passage of the internal combustion engine and configured to selectively reduce NO x contained in exhaust gas when a reducing agent is supplied thereto; a supply apparatus that is configured to supply the reducing agent to the selective reduction type NO x catalyst; a controller configured to: calculate a reduction consumption amount, which is an amount of the reducing agent amount consumed per unit time by the selective reduction type NO x catalyst for reduction of the NO x; calculate a desorption amount, which is an amount of the reducing agent, from among the reducing agent adsorbed to the selective reduction type NO x catalyst, that is desorbed from the selective reduction type NO x catalyst per unit time without being consumed for reduction of the NO x, and when the reducing agent adsorption amount adsorbed to the selective reduction type NO x catalyst is held at the target adsorption amount, control a reducing agent supply amount supplied from the supply apparatus per unit time to an amount obtained by adding the calculated reduction consumption amount and desorption amount. calculate a reduction consumption amount, which is an amount of the reducing agent amount consumed per unit time by the selective reduction type NO x catalyst for reduction of the NO x; calculate a desorption amount, which is an amount of the reducing agent, from among the reducing agent adsorbed to the selective reduction type NO x catalyst, that is desorbed from the selective reduction type NO x catalyst per unit time without being consumed for reduction of the NO x, and when the reducing agent adsorption amount adsorbed to the selective reduction type NO x catalyst is held at the target adsorption amount, control a reducing agent supply amount supplied from the supply apparatus per unit time to an amount obtained by adding the calculated reduction consumption amount and desorption amount.