The National Institute of Information and Communications Technology (NICT), the University of Tokyo Graduate School of Engineering, Sony Computer Science Laboratories, Next Generation Space Systems Technology Research Association, and SKY Perfect JSAT are collaborating on a transportable optical terrestrial system from the International Space Station (ISS) in low orbit to the ground. Through optical communication to the station, a secret key of more than 1 million bits was shared in a single overflight pass, successfully demonstrating secure communication between the ISS and the ground station in terms of information theory. We have developed a technological outlook for sharing high-speed and highly secure encryption keys with any ground station through optical communication from low-orbit satellites.