PURPOSE: To smoothly carry out the inclined rotation of a holder member, by forming a semi-spherical jaw at a head portion of a joint pin, and making the jaw abut against a seat formed in a through-hole.

CONSTITUTION: A joint pin 6 is formed at its one end with a head portion 6a, and is located in a through-hole 1b formed in a joint member 1. The head portion 6a is formed with a jaw having a semi-spherical surface 6a'. The semi- spherical surface 6a' is seated on a conical surface 7a of a guide bush 7 engaged in the through-hole 1a of the joint member 1. Accordingly, when a working reaction is applied to a tool, there is generated a force for rotating the holder member 4 to effect a smooth inclined rotation of the holder member 4 about a central point X of the semi-spherical surface 6a' of the head portion 6a of the joint pin 6.