This paper deals with a computational method which enables us to predict the movements of arbitrarily-shaped objects floating on free-surface flows. The objects are modeled as rigid bodies represented by tetrahedron elements. The present method is applicable to non-potential flows, since it is based on the MICS, which is a computational method for incompressible multiphase fluids. It was shown that natural frequencies and stability of the initial attitudes of floating objects are successfully predicted by this method and that it is also effective to predict the movements of floating objects in wave flows.