Abstract: This invention relates to an offshore unit structure. When the sea bottom ground on which an offshore structure is to be built is weak, in order to prevent the structure from being deformed by gravitational load of the structure or by horizontal load applied thereto, a cement type solidifying agent is impregnated into and mixed with subsoil in such weak ground to construct a unit-volume reformed ground, and a unit structure body is built upon such reformed ground. The structure according to this invention can be advantageously used for prospecting or producing oil in the sea.