PURPOSE: To prevent dust which is generated during chucking operation, by providing a magnetic fluid seal member in a part exposed to the outside, among parts in which a rotary arm, a support shaft, a rotary shaft and a support member make slide-contact with each other on rotation.

CONSTITUTION: Each of rotational slide contact parts which are dust generating sources, that is, a part in which a rotary arm 3 is fitted in a stationary cylinder 6, a part in which a rotary shaft 4 is supported by a bearing 13, and a part which is exposed to a part in which a support shaft 5a is supported by a bearing 21, is provided with a magnetic fluid seal member 25. With this arrangement, it is possible to eliminate such a risk that produced dust scattering toward a chuck plate 1 and a disk-like member (semiconductor wafer), thereby it is possible to maintain the disc-like member in a clean condition.