PURPOSE: To facilitate a job for lifting control over the static position of a mid floor and a load movement by installing a chain and a wire rope jointly in an underwater mid floor go up and down.

CONSTITUTION: A mid floor 3 attached tight to a frame 7 is suspended by a lifting gear 6 in a vertical groove 5 installed in a pool wall. This lifting gear 6 is provided with two fulcrum shafts 11 horizontally and parallelly at both vertical positions of the vertical groove 5, and each two of sprockets 12 and pulleys 13 are jointly installed in these fulcrum shafts 11. In addition, a chain 14 is spread over both the vertical sprockets 12, while a pool side of this chain 14 is attached tight to the tip of a projecting arm 10 of the frame 7, and drive by the chain 14 is use for only the lifting motion of the mid floor 3. Next, a wire rope 15 is spread over an interval between two pulleys 13 and 13, and one end of the pool side is locked to the upper and lower parts of a projecting arm of the mid floor via a bracket 25, and further a wire rope clamping device 25 is installed in the vertical groove 5, through which the mid floor 3 is locked to by the wire rope 15 in a state of going up and down to the desired position through the chain 14, and then its fine adjsutments are carried out by the chain 14.