To clarify the angle direction of a cutter, and keep the angle of the cutter constant to finish a sharp cutter and make right and left cutters uniform by providing an angle line on a flit surface part of the cover.

A taper rod is inserted in a gap between a guide bar 11 and a chain 13 from a taper rod opening part to eliminate looseness of the chain 13. The forward end part of a cover 1 is pressed to a member fixed not to be moved to the opposite side, or a granular part 7 is received by a rubber-soled sock or an ankle not to be moved to the opposite side, and a round file is pressed along the angle line 10 of a flat surface part 9 provided inside the cover 1 to perform dressing. At the time of performing for the opposite cutter 12, the cover 1 is left as it is, and it is turned to file along the angle line 10 in its direction.