PURPOSE: To stably hold a holder by a tool pot by building a truncated conical top the side surface energized by an elastic body toward the holder of which is inclined toward the holder and a steel ball which can be projected from the side surface of an insertion cylinder as it is pressed by the side surface in the insertion cylinder fit in the tool pot.

CONSTITUTION: A holding mechanism of a holder 10 presses a hollow tapered part 9 by a steel ball 13 from inside. When the holder 10 and a tool pot 12 are engaged with each other, an insertion cylinder 16 is inserted into the hollow tapered part 9. The steel ball 13 of the insertion cylinder 16 is constantly energized in the radial direction and the holder direction by the side surface of a top 18, but the steel ball 13 is not projected on the tapered inside surface except for a hooking part 8 as it is restrained by this side surface. However, when the steel ball 13 is positioned at the hooking part 8, it is projected from the side surface of the insertion cylinder 16, and this hooking part 8 hooks the steel ball 13. Additionally, the outside surface of the insertion cylinder 16 slides on and makes contact with the inside surface of the tapered part 9, and when the outside surface of the tapered part is made to slide on and make contact with the inside surface of the tool pot through a tapered sleeve 15, the holder 10 is stably held.