PURPOSE: To form a corrugated-plate filter medium into a provisional cylinder by pushing the filter medium by a pusher into a forming cylinder to the pushing end point position from the insertion port provided on the peripheral side of the cylinder and pushing the end part of the filter medium remaining between the pusher and the insertion port into the cylinder.

CONSTITUTION: A filter medium pusher 5 is arranged on one side of a holder 4 for a corrugated-plate filter medium A. A forming cylinder 6 having a cylindrical inner face and provided with an insertion port 10 for the medium A on its peripheral side communicating tangentially with the pushing path of the pusher 5 is arranged on the other side of the holder 4 with its axis orthogonal to the pushing path. The rear end of the medium A remaining between the pusher 5 and the insertion port 10 is pushed into the cylinder 6 by a pushing member 11 confronted with the lower side of the pusher 5. The inserted rear end of the medium A is pressed on the tip of the medium A in the cylinder 6 by a pressing member 12. Consequently, the medium A is formed into a provisional cylinder.