PURPOSE: To prevent a rotary shaft from making into eccentric contact with the inner surface of a seal member fitted rotatably therein with the rotary shaft even though the latter deflects, by disposing the seal member movably in a housing through the intermediary of a resilient seal member.

CONSTITUTION: When vacuum pressure is fed into a housing 5, a base plate chuck 3 sucks up under vacuum a base plate 16 as a workpiece through a suction hole 2a in a spin shaft 2. At this time, the atmospheric pressure is effected through the gap 15 between a spin motor 1 and the housing 5, and therefore, the seal member 6 is urged along the spin shaft 2 while it is self- aligned so that an O-ring is deformed for sealing. Then, when a spin shaft 2 is rotated so that the shaft is deflected, the seal member 6 is displaced radially to elastically deform the O-ring 7 to prevent eccentric contact. Thus, the deflection of the rotary shaft may be absorbed to sustain the sealing effect appropriately.