Response characteristics of semi-submersible type VLFS are discussed for optimization of the structural configuration. In this paper column diameter, column spacing, deck rigidity etc. are treated as components consisting the structural configuration. The problem is qualitatively discussed by use of results derived from simple analytical method and quantitatively estimated by numerical analysis. Semi-submersible type VLFS which is supported on many columns of large diameter is rationalized for large scale use such as air-port application from the view point of response characteristics through parametric studies and tank test. We also discuss design methodology of semi-submersible type VLFS. The most dominant parameter governing its response characteristics was proved to be column configuration, that is, the ratio of column diameter to length between columns, which is related to restoring force.