This study aims at development of a cost-effective floating offshore wind turbine. The proto-type model considered herein is composed of 1) 2MW horizontal-axis wind turbine (HAWT) of down-wind type, 2) steel mono-tower with 55m hub height above sea level, 3) steel-prestressed concrete (PC) hybrid SPAR-type foundation with 70m draught, and 4) catenary mooring system using anchor chains. In order to demonstrate the feasibility of the concept, on-sea experiment using a 1/10 scale model of the prototype has been made. The demonstrative experiment includes 1) construction of the hybrid SPAR foundation using PC and steel as same as the prototype, 2) drytowing and installation to the on-sea site at 30m distance from the quay of the Sasebo shipbuilding yard, 3) generating electric power using a 1kW HAWT, and 4) removal from the site. During the on-sea experiment, wind speed, wind direction, tidal height, wave height, motion of the SPAR, tension in a mooring chain, and strains in the tower and the SPAR foundation have been measured. Motion of the SPAR has been numerically simulated and compared with the measured values, where basically good agreement is observed.