Low frequency added mass of a semi-submersible is increased by incident waves, similarly to low frequency damping. It can be 1.6 times value for still water and affects the resonant frequency of the slow drift motion as well as the amplitude of it. This is a very interesting phenomenon and there is no physical explanation to explain this yet. In order to investigate this phenomenon forced oscillation tests in regular waves were carried out not only for a total semi-submersible model but also several constituent models, those being the lowerhull, brace, column, column lowerhull, columm brace, column brace of 2/3 narrowed breadth, column brace of 2/3 shortened length. It is important to elucidate this phenomenon on the design of mooring of a floating body. The experiments show that the brace plays the most important role of the change of added mass of the semi-submersible model, and that the added mass increases proportionally to square of wave amplitude of incident waves rather than the wave amplitude itself.