PURPOSE: To increase the number of degrees of freedom for position selection of an auxiliary contact by providing the auxiliary contact and forming a contacting face, which is brought into contact with the auxiliary contact when a main contact passes the guide face, in a face other than the guide face of the main contact.

CONSTITUTION: Scales 1 are connected by a joint 18, and a main contact 4 is brought into contact with a position 4A on the scale 1, and an auxiliary contact 5 is brought into contact with a position 5A. The gap between the surface of the scale 1 and the contact 5 is equal to the thickness of a parallel plane glass member 7, and the member 7 is arranged across the joint 18. A detector 3 scans the scale 1, and the length in the scanning direction of the detector 3 is so determined that the auxiliary contact 5 rides on the member 7 before the main contact 4 reaches the joint 18 and the auxiliary contact 5 is separated from the member 7 after the main contact 4 gets over the joint 18. By this constitution, the number of degrees of freedom for position selection of the auxiliary contact is increased for design.