PURPOSE: To hold a short tapered holder positively with a small number of parts by forming the semicircular hole provided part of a tool pot body and a pressing member of steel balls and a compression spring integrally combined and insert-fitted in the hole provided part.

CONSTITUTION: Three steel balls (b) energized by the resilience of a compression spring (a) pressing in the direction of an outer diameter are brought into contact with a small diameter part 7d provided at the inner diameter part of a short tapered holder 71, and the steel balls (b) are pressed in the inner diameter direction of a tool pot body 21. With the advance of the short tapered holder 71, the tapered face 7a of the holder 71 is insert-fitted to the tapered face 2a of the tool pot body 21. At this time, the short tapered holder 71 is enclosed in the state of three steel balls (b) energizing to hold the inclined face 7c 6f the short tapered holder 71 to the smell diameter side of the tapered face 2a of the tool port body 21 by the resilience of the compression spring (a).