To insure reliability in strength without causing increase of weight and cost by supporting a hook lever so as not to generate large bending stress on the hook lever holding the rotational moving shaft of a fin in the locked condition, even if excessive large fluid force is applied to the fin under the condition in which the fin is rotationally moved and projected outboard.

When the rotationally moving shaft 1a of a fin rotationally moving and projecting outboard from the inside of the ship side part in the use is driven by a hydraulic cylinder 4 into the fin projecting condition, one side 6a of the extreme end part of a derricking rotationally moving type hook lever 6 is brought in contact with the engaging part 3a projecting from the rotationally moving shaft 1a, and the other side 6b is supported with a carrier fitted to a hull 10. Hereby, even if strong fluid force is applied to the fin projecting outboard, bending stress is not generated on the hook lever 6.