PURPOSE: To ensure waterproofing by furnishing a seal, which seals a capstan relative to an outer cylinder while contacting its top when the capstan sinks to the accommodation position.

CONSTITUTION: A disc-shaped seal plate 35 is fixed to the top of a capstan 28, and a ring-shaped rubber packing 36 mating with the top of the outer cylinder 4 is installed on the undersurface of the periphery of this seal plate 35 overhung out of the capstan 28. In case the capstan 28 is out of service, an inner cylinder 18 is sunk by a hydraulic cylinder 15 to the accommodation position, and the capstan 28 retracts to under the deck 1. A stopper 11 is moved to the working place to be fitted in an upper stopper receiver 25 of the inner cylinder 18, which is thus fixed to the accommodation position. The packing 36 on the capstan 28 is put in pressure contact with the top of the outer cylinder 4 to ensure waterproofness and withstand pressure.