PURPOSE: To detect the load of an arm without adding a special force sensor besides arm driving mechanism by providing sensor mechanism in a reduction gear.

CONSTITUTION: With a motor 12 as drive source, a small diameter gear 1 on the driving side and a large diameter gear 2 on the driven side are meshed with each other to obtain a high reduction gear ratio, and an arm 8 is turned at appropriate speed. A radial beam is formed around the rotary shaft of the large diameter gear 2 on the driven side, and a distortion gage 3 is fitted to this beam so as to electrically detect the distortion of the beam caused by the difference between driving torque and load torque. Robot mechanism can be thereby made compact and lightened.