A floating offshore airport, a floating offshore city, etc. are giant marine structures supported by a great number of floating bodies as never been experienced in the past. Because of limitations in scale and ability of an experimental tank and a model, it very difficult to experimentally estimate the wave exciting forces and the drifting forces acting on these structures. It is necessary to establish a theoretical method for their accurate and practical estimation. For this purpose, we adopt, as a floating element, a semi-submersible circular dock with a cylinder type footing (Fig. 1) which will be actualized as a column of marine structures as mentioned above, and derive a theoretical formula from the velocity potentials for determining the wave exciting forces and the drifting forces acting on the floating elements which are arrayed in many rows and an infinite number of columns. In development of the theory, we divide a fluid domain into three parts, assume the velocity potentials in each part to be infinite series expansion of eigenfunction in each part, and take only the diverging waves interaction into account between rows and columns of floating element. A feature of this theoretical formula is that an interaction effect between rows can be explicity obtained. The numerical values obtained from this formula are in good agreement with those obtained by experiment.