PURPOSE: To stably install on a water bottom.

CONSTITUTION: A hydraulic cylinder 1401 is equipped at each leg 14 extending downward from the four corners of a square base 11 positioned on the bottom 3 of the sea. A grounding piece 1403 is attached at the front end of piston rod thereof. The distance from the grounding piece 1403 to the bottom 3 of the sea is measured by a submarine sensor 16 buried in each grounding piece 1403 and, in accordance with the measured result, the piston rod of hydraulic cylinder 1401 is slid appropriately by the control of a control box 18. Each leg 14 is extended or contracted so that respective grounding pieces 1403 get in contact with the bottom 3 of the sea.