To obtain a sufficient vibration damping effect while suppressing inclination of a rotary tool when the rotary tool comes into partial contact with a surface to be machined.

A grinding wheel 20 as a rotary tool is detachably attached to the spindle of a power tool by a rotary tool holder 21. The rotary tool holder 21 includes: a lock nut 25 attached to the spindle; a wheel washer 27 facing the back surface side of the lock nut 25; and an elastic body 36 disposed between the front surface of the wheel washer 27 and the back surface of the spindle wheel 20. The elastic body 36 has a first region which is compressed in the opposing directions of the wheel washer front surface and the rotary tool back surface, and a second region which is not compressed in the opposing directions or in which at least the level of compression in the opposing direction is less than the level of compression in the first region. The second region is disposed on more outside than the first region in the diameter direction of the elastic body 36.