Transmission mechanisms such as belts and link-ages are adopted in quadruped robots for small inertia. In addition, to obtain the high torque required for legged robots, the electric motor adopts a reduction mechanism, such as a harmonic reducer or a planetary reducer. A harmonic reducer is widely used in legged robots because of its high reduction ratio and compact size. By combining the reduction and transmission mechanisms, light and compact leg designs can be realized by reducing their volume and weight. In this study, to decrease the weight and volume of legged robots, a novel mechanism that combines a harmonic reducer and belt-pulley transmission units is proposed. Furthermore, the working principles of this mechanism are elucidated and, the effectiveness is verified using 3D printed testbed with encoders. The feasibility of the proposed mechanism is confirmed experimentally, and the results obtained from the experiment are discussed from theoretical and practical perspectives.