A filter backwashing unit is disposed in a path in which process gas flows to remove at least a part of trapped dust included in a process gas by backwashing an element of a filter device that traps the dust when a process gas passes. The filter backwashing unit includes a gas injection device disposed downstream of the element in a flow direction of a process gas to inject backwashing gas toward the element from downstream; a parameter detection device configured to detect a parameter used for determination of a state of dust adhering to the element; and a control device configured to estimate a thickness of dust deposited on a surface of the element upstream of a process gas based on a result of the detection, and determine an interval at which the element is backwashed based on the estimated thickness of the dust.
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