A method of removing ash from a wall flow type particulate filter which is arranged in an exhaust passage of an internal combustion engine in which combustion is performed in an excess of oxygen for trapping particulate matter in exhaust gas, using a solid acid which is carried on the particulate filter, renders the state of the particulate filter a state where the exhaust gas which flows into the particulate filter is lowered in concentration of oxygen and the particulate filter is raised in temperature, and then renders the state of the particulate filter a state where the exhaust gas which flows into the particulate filter in an oxidizing atmosphere contains SOx.