PURPOSE: To prevent intrusion of foreign particles between slide surfaces by removing crystalline foreign matters from boundary surfaces of a slip ring and a clamp ring through disposing recessed portions in a region at the axis side of the boundary surfaces.

CONSTITUTION: A boundary surface 4 is formed in a circular shape and extends perpendicular to an axis 1. A slip ring 5 is pressed against the boundary surface 4 of a clamp ring 3 by a spring force. The boundary surface 4 of the clamp ring 3 has an axially recessed portion 6. The recessed portion 6 is located at the range radially extending up to a middle portion of the slip ring 5. Consequently, crystalline silicon oxide deposited on an inner surface of the clamp ring 3 facing the axis 1 at high temperature is kept away at a distance from the friction contact boundary surfaces of the slip ring 5 and the clamp ring 3. Thus, intrusion of the crystalline silicon oxide into this region is surely prevented. The deposits stick to the recessed portion 6 of the clamp ring 3 or along an axially elongated surface 11 of the slip ring 5.