PURPOSE: To accomlish chamfering a very small hole without bias and correctly by floating a chamfering tool only without floating a chamfering shaft holding the chamfering tool in a chamfering device for chamfering a prepared hole of a workpiece.

CONSTITUTION: In order to directly apply rotation and oscillation to a chamfering tool 28 of a chamfering device, the inner both ends of a blade portion 29 and the opposite coupling 33 are, as shown in the attached drawing, machined to width across flats, and a floating pivot 34 serving as a supporting point of oscillation of the chamfering tool 28 is pressed in the central portion thereof. A driving shaft 37 for rotating a coupling 33 is inserted in the center of the upper end of the coupling 33, and further a compression spring 35 for applying cutting pressure and a regulating plate 36 for regulating the spring pressure are incorporated round the driving shaft 37 in a flange portion of the coupling 33. The chamfering amount to be machined is determined by the length from the lower surface of a thrust bush 30 pressed in the chamfering tool 29 to the edge of the chamfering tool 29.