To provide a buffer device of a disc type rotary tool having a high buffer function with a simple structure.

A receiving piece 10 of a small diameter and a fastening nut 11 are faced each other and substantially coaxially provided on a rotary shaft 7. The receiving piece 10 is provided to be relatively non-rotatable to the rotary shaft 7. The fastening nut 11 is provided to be screwable into the rotary shaft 7. Ring-shaped holding grooves 10a and 11a are formed in a surface relative to the receiving frame 10 and the fastening nut 11. Each O-ring 12 and 13 is fitted into each of the holding grooves 10a and 11a. A part of each O-ring 12 and 13 is projected outward from each of the holding grooves 10a and 11a. This disc type rotary tool 20 of a large diameter is arranged between the receiving piece 10 and the fastening nut 11 and is fitted to the rotary shaft 7. The fastening nut 11 is screwed into the rotary shaft 7 and the rotary tool 20 is elastically held by each O-ring 12 and 13.