Annular grooves are formed on both end surfaces of a recording drum to be rotated, and two balance weights are disposed inside each of the annular grooves. The balance weight is composed of a magnet, and is attracted to the end surface of the recording drum by its magnetic force. The two balance weights are arranged at a predetermined attaching angle such that an unbalance force caused by centrifugal forces developed by a plate and a movable clamp, for example, is canceled. Every time the size and the mass of the plate are changed, the positions of the two balance weights are adjusted, so that forced vibration in the recording drum is restrained.