An engine exhaust passage has an SOx trapping catalyst (11), a particulate filter (13) holding an NOx storing/reducing catalyst, and an NOx storing/reducing catalyst (15) forming a post-treatment system and a fuel feed valve (17) for feeding fuel for post processing in the post-treatment system arranged inside it. When any catalyst has a degree of degradation exceeding a predetermined degree of degradation, the method of feeding the post-treatment fuel is reset so that the post-treatment fuel required for purification of the exhaust gas can be fed into the catalyst with the lowest degree of degradation to make the catalyst with the lowest degree of degradation handle the purification action of the exhaust gas.