A particulate filter for trapping the particulate matter which is contained in the exhaust gas is provided inside an engine exhaust passage. Additional fuel is secondarily injected from a fuel injector in an engine expansion stroke or exhaust stroke or hydrocarbons are secondarily added from an addition valve which is provided upstream of the particulate filter in the exhaust pipe. An amount of hydrocarbons which come from the fuel injector or addition valve and then adhere in the form of a liquid to the inflow end of the particulate filter, and an amount of particulate matter which reaches the inflow end of the particulate filter are respectively estimated. A degree of clogging at the inflow end of the particulate filter is estimated based on the amount of hydrocarbons and the amount of particulate matter.