An internal combustion engine is provided in its exhaust system with a particulate filter for trapping combustible particulates contained in exhaust gas. A combustion promoting material injection device is provided for injecting a material for promoting combustion of the combustible particulates into the exhaust system upstream of the particulate filter. Further there are provided a bypass exhaust passage which bypasses the combustion promoting material injection device and the particulate filter, and a flow control valve which controls the amount of exhaust gas flowing through the bypass exhaust passage to control the amount of exhaust gas flowing into the particulate filter when said combustion promoting material is to be injected from the injecting device. When the combustion promoting material is injected, the flow control valve controls the amount of exhaust gas flowing into the particulate filter so that the combustion promoting material can uniformly adhere to the surface of the combustible particulates over the entire area thereof and so that the combustion temperature of the combustible particulates is prevented from rising abnormally high.