PURPOSE: To prevent wear of contact type seal by using two kinds of seals and making one of contact type seals movable in the direction of a driving shaft, and floating up the seal when rotating the bowl.

CONSTITUTION: An annular fixed seal member 9 is attached around a driving shaft 6, and an annular movable seal member 11 having an O-ring is attached slidably to the driving shaft 6. When operation is stopped, the movable seal member 11 is brought into pressure contact with the fixed seal member 9 and shaft sealed. During operation, a spring 14 of the movable seal member 11 is swelled out by centrifugal force acting on a weight 13, and the movable seal member goes up and separated from the fixed seal member 9. However, shaft sealing is maintained by the action of a non-contact seal. Thus, little wear of the contact type seal is caused.