In an exhaust gas purification system for an internal combustion engine, a NOx selective reduction catalyst for reducing NOx with ammonia that is adsorbed on the NOx selective reduction catalyst is disposed in an exhaust passage of the engine. In the NOx selective reduction catalyst, ammonia in a first adsorption state that is adsorbed at a low temperature and ammonia in a second adsorption state that is adsorbed at a high temperature are present. Because ammonia, which has been in the first adsorption state and which is desorbed from the NOx selective reduction catalyst, tends to be discharged to the atmosphere, a control section that estimates an adsorption amount of ammonia in the first adsorption state is provided in the exhaust gas purification system for the internal combustion engine in order to control the adsorption amount of ammonia in the first adsorption state within a specified level.