An object of the invention is to realize a method of determining an abnormality in a particulate filter (5) capable of trapping and oxidizing particulate matter contained in exhaust gas in which even a minor abnormality in which small quantity of particulate matter can pass through the particulate filter (5) can be determined with a high degree of accuracy. To achieve this object, the abnormality determination method according to the invention, the transition of the inflowing exhaust gas temperature in and the outflowing exhaust gas temperature out of the particulate filter (5) during deceleration operating, and the degree of decrease in the outflowing exhaust gas temperature relative to the inflowing exhaust gas temperature is computed based on the transition. If the degree of decrease exceeds a threshold value, it is determined that the particulate filter (5) is abnormal.