This paper proposes a new efficient motion conversion system which can be used in an energy harvesting system that converts wasted kinematic energy into electrical energy. In the proposed system, a reciprocating translational motion will be converted into one-directional rotational motion that spins a generator. The system will be devised with a two overlapping chambers (chamber 1 and 2) which move relatively through the sliding joint, and a pair of flexible strings (belt, steel wire, or chain) run around the rotor of the generator. Each end of the string fixed to chamber 1 is designed not to interfere with chamber 2 where the generator is mounted. When the two chambers move relatively, either top or bottom string is tensioned to spin the rotor while the other string is being rewound. One-directional clutch with a coil spring is engaged in a rewinding system – as found in a rowing machine, for example – so each string actuates the rotor only when it is in tension. This device can be applied to any mechanism where reciprocating translational motion exists, such as linear suspension system in a vehicle, a bicycle, and an energy generating marine buoy. The experimental study result will be reported as well as its battery-charging capacity will be demonstrated.