To securely hold a tool by improving operability for inserting the tool and reducing the weight.

A wide width part 12 which is wide in the radial direction is formed at a central part in the longitudinal direction of a service hole 11, and a storage part 13 in which a pin 7b of a tool is stored is formed at an end in the longitudinal direction of the service hole 11. The pin 7b is inserted in the wide width part 12 of the service hole 11, and when relative rotation to the tool is generated, the pin 7b is stored in the storage part 13 of the service hole 11.