Approaches to the overall structural response analysis of a semi-submersible offshore structure in waves are widely divided into two, i. e. the deterministic method based on design wave and the stochastic method based on statistical prediction. At present, the former, the deterministic method based on design wave, is more often applied to the design of an actual structure than the latter from the viewpoints of simplicity in computation, etc. In application of the former, it is also important to study non-linearity in response (effect of wave height) in very high waves. This paper first presents the results of a tank test that was performed to verify the applicability to practical problems of the approximate overall structural response analysis method in high regular waves based on the finite amplitude and non-linearized theory. In the tank test, a 1/50-scale structural model of a 2-lower hull and 8-column type semi-submersible offshore structure was used. Subsequently, a prototype model of a semi-submersible offshore structure was used to investigate the non-linearity in response (effect of wave height) in very high waves by performing a numerical analysis, and the degree of its significance lying in the design of an actual structure was also discussed.