A floating vertical circular cylinder in regular waves is dealt with as a model of analysis of the behaviour of floating offshore structures. Wave forces on a fixed cylinder and movements of a freely floating one in regular waves are calculated by three methods: the strict method by Ijima based on linear potential theory, the approximate method by Ito which neglects the terms of infinite series corresponding to the exponentially diminishing stationary waves, and the strip method with hydrodynamic coefficients obtained from model experiments. The results of these methods are compared with the data of measurements of wave forces, heaving,swaying, and drifting for five models with the diameters of 10 cm and 20 cm in the water of 40 cm deep, and then the accuracy of these methods is examined.
　Among the movements of floating cylinder, drifting is an interesting one. Experiments have shown that the drifting velocity may exceed 80% of the maximum horizontal orbital velocity. The drifting velocity has been calculated by consideration of the nonlinear effects between the wave pressure and the movements of the floating body. The drifting velocity is estimated from the drifting force by the assumption that it is balanced with hydrodynamic resistance force in proportion to the square of drifting velocity.