Authors have been developing a pure mechanical wave power generation system. The principle of the system is the application of the gyroscopic moment produced by the rotation of large flywheels and the swing of a float excited by wave motion. This paper presents the principle of the system, the developing process of prototype systems and their test results. It was confirmed that the system has higher efficiency than conventional OWC wave power generation systems and sufficient reliability for actual application. The system will be applicable for distributed electric source for small island and ports.