PURPOSE: To mount a buffering material to the top of a divided socket without using any bolt by installing a key-type engaging part, which can be engaged with each other, to an upper part inner surface of each boss block and a lower part outer surface of each buffering material respectively and threading a boss block into a second kelly-bar.

CONSTITUTION: An engaging part 7a of a boss block 7A is engaged with a pair of engaging parts 8a of a buffering material 8, thereby constituting a composite block. The inner peripheral surface of this composite block is placed into contact with the outer peripheral surface of a second kelly-bar 2 while a bolt penetration hole 7c of the boss block 7A is adapted to communicate with a bolt hole 2a of the second kelly-bar 2. A bolt 21 is inserted into the penetration hole 7c so as to fix the composite block with the second kelly-bar 2. Then, the other two composite blocks are mounted to the lower part outer peripheral surface of the second kelly-bar 2 in the same manner while the lower part outer peripheral surface is covered with a divided boss 7. Three buffering materials 8 are laid out on the upper part in the shape of a ring.