PURPOSE: To facilitate operability for attaching and detaching an outer blade part to/from a main body, to prevent the damage of an outer blade frame at the time of attachment and detachment and to compact the outer blade frame by increasing flexibility at an upper part on the end face of an outer blade frame body.

CONSTITUTION: An elastic part 8 for locking is provided with a hook 14 where notched recessed parts 13 are formed on both sides and the hook 14 is provided with elasticity and forms a sufficiently flexible shape. Further, on the end face 3a of the outer blade frame body 3, a notched part 3c is formed along the inner side of the side wall 3b of the outer blade frame body 3. Thus, since the upper part of the end face 3a is provided with sufficient flexibility, in the case of releasing the hook 14 from the outer blade frame, bending moment acts on the part of a line segment 3d and the attachment and the detachment are performed by the synthetic deformation of the hook 14 and the side wall. Thus, the stress of the root of the hook 14 is mitigated and turned to the allowable stress even when the hook is not so long and the outer blade frame body is compacted.