A pair of particulate filters arranged in parallel in an exhaust passage and a combustion device for supplying to the filters exhaust gas having a temperature elevation capability higher than the temperature elevation capability of the engine exhaust gas able to raise the filter temperature are provided. An NOx catalyst and oxidizing substance are carried on each filter. When the NOx absorbed in an NOx catalyst should be reduced, the flow of engine exhaust gas into the filter carrying the NOx catalyst for reduction of the NOx is suppressed and a rich air-fuel ratio exhaust gas is supplied from the combustion device to the filter so that the filter temperature is maintained higher than a particulate continuous oxidation temperature in the NOx reduction temperature range.