This report is about a numerical calculation method for the wave energy absorption of a floating 2D cylinder. The least condition for the complete energy absorption by making use of an oscillating cylinder in waves is that the floating cylinder has to be wave-free on one side direction when it is forced to oscillate in a given motion mode. The radiated wave amplitude can be expressed by the Kochin function. Based on the relation between the small deformation of the floating body’s profile and the Kochin function, the numerical calculation method which satisfies the complete energy absorption condition by changing the configuration step by step is derived. Some numerical examples are demonstrated.