PURPOSE: To easily change only the blade by providing a blade insert-through part to be inserted through a holding groove and a blade on a rotating body base and providing an end face plate for closing the holding groove on the rotatating body.

CONSTITUTION: At the time of changing a blade 35, an end face plate 56 is rotated in the direction as indicated with an arrow A so that a projecting part 62 of the end face plate 56 engaged to a recessed part 47 of a shaft holding body 41 is engaged to a recessed part 46. Subsequently, a blade insert-through part 19 is positioned on a production of a holding groove 32 so as to allow the blade insert-through part 19 and the holding groove 32 to communicate with each other. Next, the blade 35 is drawn out in the direction as indicated with an arrow B from the holding groove 32, and a new blade 35 is inserted into the holding groove 32 in the direction opposite to the arrow B. Thereafter, the end face plate 56 is turned in the direction opposite to the arrow A, and the projecting part 62 is engaged to the recessed part 47 again and the holding groove is closed by a closing wall 58, and a protruding part 36 of the blade 35 fitted newly is detained in a state that it comes into contact with the closing wall 58. In such a way, only the blade 35 can be changed easily.