A particulate filter (22) is arranged in an exhaust passage of an engine, while an exhaust throttle valve (45) is arranged in the exhaust passage downstream of the particulate filter (22). The exhaust throttle valve (45) is fully closed once, then fully opened cyclically. At that time, the flow velocity of the exhaust gas is increased for just an instant in a pulse-like manner, whereby masses of particulate are separated from the particulate filter 22 and discharged.