An exhaust gas purification apparatus for an internal combustion engine, comprising: a NOx storage/reduction catalyst, provided in an exhaust passage of the internal combustion engine, configured to store NOx when an air-fuel ratio of exhaust gas is larger than a stoichiometric air-fuel ratio and to reduce the stored NOx when the air-fuel ratio of the exhaust gas is equal to or smaller than the stoichiometric air-fuel ratio; a NOx selective reduction catalyst, provided in the exhaust passage downstream of the NOx storage/reduction catalyst, configured to reduce NOx using ammonia as a reducing agent; a NOx sensor for detecting concentration of NOx and ammonia in the exhaust gas in the exhaust passage downstream of the NOx storage/reduction catalyst and upstream of the NOx selective reduction catalyst; an air-fuel ratio decrease unit configured for setting the air-fuel ratio of the exhaust gas flowing into the NOx storage/reduction catalyst at or below the stoichiometric air-fuel ratio; and an ammonia supply apparatus for supplying ammonia from an upstream side of the NOx selective reduction catalyst, characterized by a control apparatus configured for determining an amount of ammonia supplied from the ammonia supply apparatus on the basis of a detection value of the NOx sensor, wherein when the air-fuel ratio of the exhaust gas flowing into the NOx storage/reduction catalyst is set at or below the stoichiometric air-fuel ratio by the air-fuel ratio decrease unit, the control apparatus is configured to make the amount of ammonia supplied from the ammonia supply apparatus, relative to the detection value of the NOx sensor, less than when the air-fuel ratio of the exhaust gas is larger than the stoichiometric air-fuel ratio, and when the air-fuel ratio of the exhaust gas flowing into the NOx storage/reduction catalyst is set at or below the stoichiometric air-fuel ratio by the air-fuel ratio decrease unit, the control apparatus is configured to lessen the amount of ammonia supplied from the ammonia supply apparatus relative to the detection value of the NOx sensor by correcting the detection value of the NOx sensor and determining the amount of ammonia supplied from the ammonia supply apparatus on the basis of the corrected detection value of the NOx sensor. a NOx storage/reduction catalyst, provided in an exhaust passage of the internal combustion engine, configured to store NOx when an air-fuel ratio of exhaust gas is larger than a stoichiometric air-fuel ratio and to reduce the stored NOx when the air-fuel ratio of the exhaust gas is equal to or smaller than the stoichiometric air-fuel ratio; a NOx selective reduction catalyst, provided in the exhaust passage downstream of the NOx storage/reduction catalyst, configured to reduce NOx using ammonia as a reducing agent; a NOx sensor for detecting concentration of NOx and ammonia in the exhaust gas in the exhaust passage downstream of the NOx storage/reduction catalyst and upstream of the NOx selective reduction catalyst; an air-fuel ratio decrease unit configured for setting the air-fuel ratio of the exhaust gas flowing into the NOx storage/reduction catalyst at or below the stoichiometric air-fuel ratio; and an ammonia supply apparatus for supplying ammonia from an upstream side of the NOx selective reduction catalyst, characterized by a control apparatus configured for determining an amount of ammonia supplied from the ammonia supply apparatus on the basis of a detection value of the NOx sensor, wherein when the air-fuel ratio of the exhaust gas flowing into the NOx storage/reduction catalyst is set at or below the stoichiometric air-fuel ratio by the air-fuel ratio decrease unit, the control apparatus is configured to make the amount of ammonia supplied from the ammonia supply apparatus, relative to the detection value of the NOx sensor, less than when the air-fuel ratio of the exhaust gas is larger than the stoichiometric air-fuel ratio, and when the air-fuel ratio of the exhaust gas flowing into the NOx storage/reduction catalyst is set at or below the stoichiometric air-fuel ratio by the air-fuel ratio decrease unit, the control apparatus is configured to lessen the amount of ammonia supplied from the ammonia supply apparatus relative to the detection value of the NOx sensor by correcting the detection value of the NOx sensor and determining the amount of ammonia supplied from the ammonia supply apparatus on the basis of the corrected detection value of the NOx sensor.