A concept on a ring-like semisubmersible for ocean space utilization of middle water region was proposed in the 1st and 2nd papers. Principal concepts of this structure are summarized by the following items : semisubmersible, ring-like shapa in plan supporting structures and connection system. The structure is anchored by supporting structures at several points and a special connection system between a semisubmersible and supporting structures is adopted, which enables such a function that the structure is almost rigidly connected with supporting structures in case of severe storm but almost completely released in case of severe earthquake. In this paper, in order to prove the above concept on the connection system, several elements of the system are properly designed and computer simulations and simplified, small scale model tests on a full connection system are carried out by using the numerical data based on the time history records of accelerations of actual severe earthquakes. The results are discussed and a way of release of the connection system is suggested to avoid transferring earthquake motions from the supporting structures to the semisubmersible.