The present invention includes: a rolled filter stretched between first and second winding shafts and, unused part of the filter being wound around the second winding shaft; a brush provided between the first and the second winding shafts so as to come into contact with a surface of the filter; a feeding mechanism for driving the first and the second winding shafts to take up or rewind the filter; and a feeding control unit for controlling the feeding mechanism. When activating a renewing operation for feeding unused part of the filter from the second winding shaft, the feeding control unit controls the feeding mechanism such that once the filter is taken up by the first winding shaft by an take-up amount La, the filter is rewound by the second winding shaft by a rewind amount Lb, and La and Lb satisfy a relationship La≧Lb. Without increasing the amount of the filter or the size of the device itself, the present invention can deliver adequate performance even in a dusty environment by a small number of filter renewing operations.