A solar sail can move forward without consuming propellant as long as it can generate enough energy from sunlight. This idea was born some 100 years ago, but it had lots of technical hurdles such as the appropriate material and deployment method for the sail. Recently, we have finally seen some prospect of using this technology practically. The sail of the IKAROS is a huge square some 20 meters in a diagonal line, as thin as 0.0075 mm, and made from polyimide resin. On the membrane of the sail are not only thin film solar cells but also an Attitude Control device and scientific observation sensors. This thin and light solar sail membrane will be deployed using the centrifugal force of spinning the main body of the IKAROS before its tension is maintained. The deployment is in two stages. The first stage is carried out quasi-statically by the onboard deployment mechanism on the side of the main body. The second stage is the dynamic deployment. As this deployment method does not require a strut such as a boom, it can contribute to making it lighter, thus can be apply for a larger membrane.