Device for purifying the exhaust gas of a diesel engine

A device for purifying the exhaust gas of a diesel engine comprises a filter for collecting particulates, which is arranged in the exhaust system of the engine; an oxidation catalyst arranged in the exhaust system upstream of the filter; diesel fuel supply means for supplying diesel fuel to the oxidation catalyst when the filter must be regenerated; intake air decreasing means for decreasing an amount of intake air in the initial step of supplying the diesel fuel; and diesel fuel decreasing means for decreasing the amount of diesel fuel supplied from the diesel fuel supply means when the amount of intake air is decreased by the intake air decreasing means.