PURPOSE: To form an annular groove of arcuate cross-section by cutting, by rotating a work by means of a spindle and circumferentially moving a cutting tool through a worm and a worm wheel for each rotation of the work.

CONSTITUTION: A work 13 is so attached that the axis of rotation thereof alignes with the axis of the spindle of a machine tool such as a boring machine. A semicircular groove 1a to be formed is formed by cutting the work 13 by a cutting tool 4 while rotating the work 13 by the spindle. A star wheel 9 makes a contact with a stopper 18 at each rotation of the work 13, and is intermittently rotated in accordance with the number of teeth 9a thereof. As a result of this intermittent rotation totation, the cutting tool 4 is bed in the circumferential direction around the shaft 7, through the medium of worm 6 and the worm wheel 9, so asf to form the desired semicircular groove 1a. The radius of this groove 1a is adjusted by changing the projection length of the cutting tool, while the diameter of the bore is adjusted by changing the posi tion of the work 13 with respect to the boring bar 19.