PURPOSE: To easily build a floating body of concrete and steel, by arranging concrete blocks with reinforcing iron protruded, in the place of the bottom board of the floating body, to dispense with specially supporting the reinforcing iron at a height required, and by then placing concrete around the blocks.

CONSTITUTION: To build the bottom board 2A of a floating body of outer concrete 2 and inner reinforcing steel 3, forms 6 for molding the bottom of the floating body are arranged on a smooth and horizontal ground 5 and concrete blocks 7 are provided in a bottom section defined by the forms 6 located in four directions, so that the steel of each concrete block is supported and reinforcing iron 9 is protruded from the block. Concrete is then placed around the concrete blocks 7 inside the forms 6. The bottom board 2A of concrete reinforced by the iron 9 is thus manufactured without specially supporting the iron 9.