To provide an underwater bottom plate constrution method of a bottomless structure, easy in construction without substantially requiring repair while surely constructing a bottom plate, by draining water, remarkably shortening the number of days up to enabling work, grasping strength of the bottom plate, and enabling a structural calculation.

A concrete bottom plate block B provided with a grout material filling promoting introduction taper 6, is prepared by arranging the proper number of wire hooking metal fittings 5, 5 and 5 on an upper surface by being formed smaller than an inside measurement of an immersed shaft A so as to generate clearance (a) between the bottom plate block and the immersed shaft A. First of all, this bottom plate block B is hung down and seated on the ground in the immersed shaft A by hooking the tip of a wire 7 of a crane on the wire hooking metal fittings 5, 5 and 5. Next, a grout material 8 is filled in the clearance (a) between the immersed shaft A and the bottom plate block B.