A power transmission system for a working machine, wherein drive power is transmitted from an engine to a work-tool holding drive shaft via a reduction mechanism is provided. The reduction mechanism includes an input shaft having a first gear, and an output shaft having a second gear. The second gear has a gear portion and a cylindrical boss segment press fitted thereto, forming a torque limiter. As a result, the reduction mechanism, which has the torque limiter, has a simplified structure, enabling easy assemblage.