PURPOSE: To certainly and efficiently shake off the dust accumulated the filter cloth of a bag filter by measuring the differential pressure of the filter cloth of the bag filter to compare the measured differential pressure value with the reference differential pressure value calculated corresponding to the flow rate of gas to be treated and shaking off dust when the measured differential pressure value exceeds a reference differential pressure value.

CONSTITUTION: A differential pressure measuring device 5 measuring the differential pressure of filter cloth 3 to detect a measured differential pressure value and the control unit 6 connected to the differential pressure measuring device 5 and comparing the measured differential pressure value with a reference differential pressure value to operate dust shaking-off treatment devices 7, 8 under a constant condition are provided. Flowmeters 9a, 9b measuring the flow rate of gas to be treated are connected to a control unit 6 and a means calculating a reference differential pressure value corresponding to the measured values of the flowmeter is provided. The differential pressure of the filter cloth 2 of a bag filter is measured and the measured differential pressure value is compared with the reference differential pressure value calculated corresponding to the flow rate of the gas to be treated and, when the measured differential pressure value exceeds a reference differential pressure value, the shaking-off treatment of dust is performed.