PURPOSE: To prevent an upper main body and a lower main body from slipping out or rotating, by installing a recessed part on the outer periphery of the lower main body, inserting an elastic member and a sphere into the recessed part, and installing a penetrating hole engaging with the sphere on the inner periphery of the upper main body.

CONSTITUTION: A lower main body 2B is inserted into an upper main body 2A. When the main body 2B is inserted into the main body 2A as far as the vicinity of a recessed part 2E, a sphere 5 is pressed by the inner periphery of the main body 2A. The upper surface of the sphere 5 compresses a spring 4 together with the sphere 5 as far as the outer peripheral surface of the main body 2B. In this state, the main body 2B is pushed into the main body 2A. When a protruding part 2C is completely fitted on a protrusion 2D, the main body 2B and the main body 2A are slightly moved in the direction perpendicular to the axis, thereby making the sphere 5 engage with the penetrating hole 2F. By this constitution, the main body 2A and the main body 2B do not slip off in the axial direction or rotate in the direction perpendicular to the aixs.