PURPOSE: To make it possible to correspond to a deep bearing layer accumulating thickly a weak stratum in the great depth of water and to provide the underwater foundation capable of being constructed economically and speedily without requiring any temporary structure such as a working scaffold, a guide frame, etc., even in a place having a swift current.

CONSTITUTION: A pile is inserted into main columns 4 of a prefabricated pier stud 1 constituted of the main columns 4 consisting of a longer pipe at large diameter reaching the surface of the water from the base of the sea-bed 11 and a plurality of skirt columns 5 consisting of a shorter pipe at large diameter rigidly connected with a plurality of stages of connection members 22 arranged to the outside of the base of the main columns and the skirt columns 5 and is penetrated to the sea-bed ground 7. A gap between the columns and pile is filled with grout, and the insides of the main columns 4 are filled with concrete.