PURPOSE: To deliver the supersonic vibration from a rotating member to the main body of the adaptor without attenuating substantially and permit to take it out as the detecting signal of an abnormal condition by forming a thin oil film between the rotating member and the main body of the adaptor which is supporting the rotating member.

CONSTITUTION: The oil, supplied from an oil feeding port 4, is fed to the supporting part of the rotating tubular body 8 from a path 25 through one series of oil supplying paths 26W29 and the thin oil film 37 is formed at this part. Subsequently, a main spindle 42 is rotated to rotate the rotary tool 16 and a cutting work is effected. When an overload is applied to the rotary tool 16 under the working, a torque limiter 17 is operated and the supersonic vibration is generated by the detaching motion thereof. The supersonic vibration is also generated when the tool 16 is broken or abutted against other bodies. These supersonic vibrations are transmitted to the main body 1 of the adaptor from the rotating tubular body 8 through the oil film 37 without being attenuated substantially. The supersonic vibrations are detected by a supersonic wave sensor 33 and the abnormality of the rotary tool 16 is detected, therefore, optimum tool protection may be effected utilizing the detecting signals.