PURPOSE: To certainly collect dust by mounting a frame body smaller than the cross-section of a duct, the duster fastened to the frame body at the leading end thereof and a traction rope.

CONSTITUTION: A suction dust collector 403 is operated to set the wind velocity in a duct 400 to about 15-20 m/sec. Subsequently, a duster A is introduced into the duct 400 from ao upstream opening 401 and ropes 3, 4 are loosened to the same degree to move the duster A through the duct 400 toward the downstream part thereof. The duster A flutters up and down as well as left and right by an air stream to repeatedly apply pressure impact to the inner wall 404 of the duct. Therefore, dust is released from the inner wall 404 of the duct. When the duster A passes through the branch part and profile transfer part of the duct 400, the rope 4 is pulled over the delivery length of the rope 3 or loosened to change the angle of a frame body 1. By this method, even small dust is released perfectly and can be blown off toward the downstream part.