Abstract: In a robot arm having a rotary encoder of incremental value type outputting signals including a signal (Z-signal) which is generated every one revolution of the rotary encoder and a rotary encoder of absolute value type, angles (.DELTA.Z, .DELTA.D) of the original positions of the respective rotary encoders with respect to an original position (E) of the robot arm are previously measured, then the arm is moved and is stopped at a position in which the Z-signal is generated, subsequently, an absolute position of the arm is detected on the basis of the angles (.DELTA.Z, .DELTA.D), and the angle between the arm and the original position (E) is calculated. The absolute position is detectable within one revolution of the rotary encoder of incremental type, and thereby an effective "return-to-origin operation" of the arm is accomplished.