PURPOSE: To protect the action of a drive means from interruption due to the adhesion of objects in the sea by providing a wedge part to lock a rocking arm to move a chock block to the chock body by which a platform is geared with the racks of legs.

CONSTITUTION: Rocking arms 7 and 8 and the first and second wedge parts 21 and 23 are provided to a chock block 6 by which a platform 2 is geared with the racks 4 of legs 1. The block 6 is moved from the platform 2 to the horizontal and vertical directions by the arms 7 and 8. Then, the block 6 is locked from up and down sides by the first wedge part 21 rotatably supported, having a spirally inclined face, and also locked from sides of the platform by the second wedge part 23. By this, the rotation shafts of the arms 7 and 8 and the first wedge part 21 can be housed in the platform 2.