As a future interplanetary propulsion system, research focuses on demonstrating the engineering feasibility of non-nuclear outerplanetary explorations. As a means of future interplanetary flight, the solar electric sail to drive ion engines with high specific impulse is attracting attention recently as a highly probable solution. The feasibility of the solar electric sail has been promoted by the advent of lightweight and extremely thin film technology. JAXA launched a Small Solar Power Sail Demonstrator "IKAROS" for technological verification.