Abstract: A floor panel having a rectangular floor base integrally with a convex prop in each corner portion and a concave portion on the upper surface of each prop. There is a height adjusting screw with a plate embedded in the concave portion of each prop, variably for adjusting the height of the support position in a predetermined range between a position lower than the upper surface of the prop and a position higher than the upper surface of the prop. A floor panel is supported by each prop of the floor base and has through holes for accessing the height adjusting screw for adjusting the support height at each corner according to the prop. In the method of manufacturing the floor base, a convex portion, which integrally projects to the upper surface side on four corners of a skin, is formed, a concave portion is formed on the lower surface of said convex portion, concrete is packed in the concave portion, thereby forming which is integral to said skin.