PURPOSE: To improve the positioning accuracy of a robot by means of feedback control as well as to check any vibration at a tip of the robot, in an articulated mechanism in the robot inclusive of a manipulator, a scalar robot or the like.

CONSTITUTION: A robot articulated mechanism consists of a rotary actuator 1 and a speed reducer 2 for decelerating a turning angle of the rotary actuator 1, and a first rotation detector 40 detecting a turning angle of an output shaft 5 is installed at the side of the output shaft 5 of the speed reducer 2. The rotary actuator 1 rotates according to a command value out of a servo control system. The rotation is relatively high speed and decelerated by the speed reducer 2 with the specified reduction gear ratio. The first rotation detector 40 detects the turning angle of the output shaft 5 decelerated, and the detected result is outputted to the servocontrol system which performs feedback control over the rotary actuator 1 on the basis of errors between output information out of the rotation detector 40 and the command value.