<p>A housing (2) is provided with an accommodation hole (11) into which a clamp object portion (10) circular in sectional view is inserted. An annular blank space (12) is arranged on the radial outside of the accommodation hole (11), and an annular partition (13) for sectioning the annular blank space (12) and the accommodation hole (11) is constituted as a part of the housing (2). A plurality of supporting holes (14) are arranged in the annular partition (13) at a regular interval in the circumferential direction, and each supporting hole (14) supports a lock member (15) movably in the radial direction. An annular drive sleeve (18) so inserted into the annular blank space (12) as to be capable vertically moving is made to engage with a plurality of lock members (15). An annular tapered space (46) is formed between the outer circumferential wall (45) of the annular blank space (12) and the drive sleeve (18) to become narrower toward the upper part. A tapered sleeve (47) is inserted into the tapered space (46) so that the tapered sleeve (47) is brought into taper-engagement with the outer circumferential wall (45).</p>
