A wave activated electric power generating system is durable even if exposed to a rainstorm, extremely simple in structure and highly efficient. A motion transfer medium rendering the rotary part of an electric generator less liable to produce slipping and a rotation transform member adapted to rotate while fitting with the motion transfer medium in movement are also provided. The system includes a buoy floating on the sea or lake so as to move up and down with the waves, and an underwater motion restrainer suspended underwater by water resistance, or submerged by its own weight on the sea or lake floor. Vertical movement of the buoy relative to the underwater motion restrainer is converted and transferred by the motion transfer medium in the form of an elongate rope vertically moving under the weight of an element to operate the rotary electric generator securely connected to the buoy.