The Mighty Whale, floating wave energy converter, has been studied and developed for the purpose of using coastal areas effectively by JAMSTEC (Japan Marine Science and Technology Center). The Mighty Whale will be installed at 1.5 km point from the coast of the Gokasyo-bay in Mie prefecture. In other words, the Mighty Whale will have to be moored in shallow water under severe environmental conditions by catenary mooring with chain. Generally, in case of catenary mooring, slowly varying oscillations occur and mooring tensions become very large. Furthermore, if floating structures are set in shallow water, mooring tensions are raised by their vertical motions. Therefore, there have been few similar mooring cases such as the Mighty Whale. In this paper, design methods of mooring system by approximate analysis, simulation analysis and model experiment are indicated, and concluded points through the design of the Mighty Whale are mentioned as follows ; (1) It is feasible to moor large floating structures in shallow water under severe environmental conditions by catenary mooring. (2) Intermediate weight on mooring line is effective in controlling displacement and mooring tensions. (3) Approximate analyses for considering slowly varying oscillations and influences of vertical motions are available to design of real structures' mooring systems. (4) Using these approximate analyses in initial design is simple and efficient, and useful for reducing time of design.