An apparatus for preventing a filter for purifying exhaust gas emitted by a vehicle diesel engine from overheating is provided. The filter filters particulate matter in exhaust gas. Particulate matter that is accumulated in the filter through filtering is burned and purified by executing a temperature increase process in which the filter is heated. The apparatus includes an electronic control unit as overheat prevention means. During the temperature increase process, the overheat prevention means executes an increase process for increasing the flow rate of exhaust gas when the filter is likely to overheat. As a result, the filter is effectively prevented from overheating.