PURPOSE: To install a concrete caisson without the needs for a huge crane by a method in which a caisson is built up by stacking an under block, over blocks, etc., and then settled.

CONSTITUTION: An under block 31 is placed on a plinth 31 on the surface of water, and over blocks B are stacked while additionally connecting iron bars 11. In case where cement mortar is filled in iron bar-receiving holes 12 and 16 and concrete caisson is settled, air inside a float 32 is first drawn out and then the caisson is lowered. When the caisson is immersed to a depth under water unitl it becomes floated by buoyancy. Then, the plinth 31 and the float 32 are removed, and the caisson is settled into the subgrade under water by packing soil and sand or filling water into the inside of the caisson.