To realize the semi submersible type mega-float (SSMF), it is very important to reduce the cost, which is higher than one of pontoon mega-float. Because the building cost leads to much of SSMF's initial cost, to reduce the structural weight is well meaning. In this study, we tried to optimize SSMF in order to minimize the structural weight, under the condition that the maximum bending stress is less than the allowable stress. We inspected the results by VODAC and got the knowledge between the structural weight and deck depth D, column radius R and column number N.