Abstract: A cutting tool for finishing a back surface of a workpiece having a hole is proposed. A blade having an equilateral triangular shape is attached to a shaft of the cutting tool. One of cutting edges formed along the sides of the blade projects transversely of the shaft. The tip of the projecting cutting edge comes to contact the workpiece's back surface. When the workpiece is rotated and the shaft is moved in the diametrical direction relative to the hole, the cutting edge tip grinds the back surface. At this time, if the shaft moves in the longitudinal direction of the workpiece, the blade creates a notch in communication with the hole on the workpiece's back surface, resulting in a stepped borehole.