PURPOSE: To elevate the accuracy of the transmission angle by making the stator section of an encoder free to connect the rotor section thereof a rotating shaft with a rigid coupling while the rotating direction of the arm thereof is contrained.

CONSTITUTION: As an encoder 2 is connected to a rotating shaft 3 with a rigid coupling 5, any angle error as transmitted to the rotor section 2a of an encoder 2 from the rotating shaft 3 can be ignored. The encoder 2 can be supported entirely on the rotating shaft 3. An arm 6 is fixed on a stator section with a lock on the stator section of an encoder 2 while the tip 6a thereof 6 contact a pin 7 on the one side thereof and is pressed with a spring 9 on the other side thereof overcoming a friction torque of the encoder 2. This eliminates play in the direction of rotation while allows the absorption of back-lash of the rotating shaft 3 by a vertical movement of the arm 6.