PURPOSE: To improve measurement precision, by extending a wire between the rotating shaft of a motor to be measured and a tension detector, and finding the relative position of the detector and the shaft and the tension of the wire while shifting the relative position in the take-up direction of the wire.

CONSTITUTION: The motor 25 to be measured is driven to wind the wire 24 around a drum 26 until the wire 24 is tensed between the drum and tension detector 23. In the tensed state, the wire 24 is taken up by the rotation of the motor 24 and the tension detector 23 is moved at an equal speed in the take-up direction. Then, the position of the tension detector 23 and the tension of the wire 24 by the tension detector 23 are detected simultaneously to detect the irregularity of the torque of the motor 25 to be measured.