PURPOSE: To enable prevention of a gap on the external circumferential surface of a drum, by providing a movable segment protrudable in the direction of an axis of the drum on a stationary segment fixed to a drum body and a movable cover plate on the movable segment.

CONSTITUTION: A stationary cover plate 17 constituting the external circumferential surface of a drum is secured on the external end face of a stationary segment 16 fixed to a drum body 15 and both the end parts in the direction of an axis of the drum of the segment 16 is provided with a movable segment 19 protrudable in the direction of the axis of the drum through a guide component 18. The titled drum is so constituted further that a movable cover plate 20 having a width in the circumferential direction and arriving at even the adjoining segment is fixed to the segment 19 and the segment 19 and the plate 20 are slided each other through a positive and reverse turns of a screw shaft 21 to be screwed into the segment 19 held by the segment 16. With this construction, generation of a gap on the external circumferential surface of the drum is eliminated at the time of expansion of a width of the drum and expansion and contraction of a drum diameter can be performed sufficiently.