A filter (10) which traps particulate matter contained in the exhaust gas of a diesel engine (20) for a vehicle is regenerated by fuel injection control. A controller (16) calculates a representative value of the operating condition of the diesel engine (20) during a latest predetermined time period, and determines the traveling condition of the vehicle on the basis of this representative value. When the representative value corresponds to a highway traveling condition, fuel injection control is performed in accordance with a pattern for burning all of the particulate matter trapped in the filter (10). Under any other conditions, fuel injection control is performed in accordance with another pattern. Hence optimum filter regeneration can be performed depending on the traveling condition of the vehicle.