Over the last decade the wind energy technology has dramatically been developing to meet the needs of the times for clean and renewable energy resources and the attributes of offshore wind energy resource have beenreceiving increasing attention. Our goal in this paper is to raise some key issues that must be evolved in order to realize a large-scale offshore energy base. The concept outlined here is one of the important technological areas identified by the “Committee of Japan Ocean Industries Technological Strategy for the 21st century”(Chairman: Prof. Koichiro Yoshida, October 1999 to March 2001) organized in JOIA (Japan Ocean Industries Association).The authors expect that the considerations presented in this paper can invite further continuous investigation to realize the projection based on cost effective offshore wind energy technologies.