Abstract: In an exhaust gas purification device for an internal combustion engine, unburned fuel components are not supplied to a catalyst to make the atmosphere around the catalyst lean during a first lean period, unburned fuel components are supplied to the catalyst to make the atmosphere around the catalyst rich during a rich period, and the supply of unburned fuel components to the catalyst is again stopped during a second lean period. The first lean period, the rich period, and the second lean period form a cycle. If the actual amount of unburned fuel components added to the catalyst is below a required addition amount when a cycle ends, the duration of the first lean period in a subsequent cycle is reduced.

Abstract: In an exhaust gas purification device for an internal combustion engine, unburned fuel components are not supplied to a catalyst to make the atmosphere around the catalyst lean during a first lean period, unburned fuel components are supplied to the catalyst to make the atmosphere around the catalyst rich during a rich period, and the supply of unburned fuel components to the catalyst is again stopped during a second lean period. The first lean period, the rich period, and the second lean period form a cycle. If the actual amount of unburned fuel components added to the catalyst is below a required addition amount when a cycle ends, the duration of the first lean period in a subsequent cycle is reduced.