PURPOSE: To securely measure an angle of twist in a simple operation by providing a body of rotation having a hole to pierce a stock to be worked in the central part, a clamp to fix the stock to be worked and a driving means to rotate the body of rotation.

CONSTITUTION: A billet as the stock to be worked is inserted into a hollow shaft 7a of the body 7 of rotation and when a position to be twisted comes to the position of a clamp chuck 8 and a hydraulic pressure is sent to a hydraulic ram 9, the clamp chuck 8 is closed and clamps the stock to be worked. Then, when an electromagnetic clutch is actuated, a driving motor 2 is connected with the clamp 8 and when the driving motor 6 rotates, the billet begins being twisted. When an angle of rotation is located at a set angle of twist as angles of rotation of a clamp 8 are monitored by pulses of an encoder tied to the clamp 8, a driving motor 2 is stopped. Thereafter, when the electromagnetic clutch 6 is opened, the stock to be worked generates a springback. Then, when the clamp 8 is opened, the angle of twist can be measured.