When machining of a circumferential groove is performed by advancing a cutting tool which is attached to a rotating shaft extending to an inner side from an outer side of a rotating work into the work in a depth direction, the rotating shaft is downwardly inclined toward the inner side of the work at a predetermined inclination angle, a trajectory of a cutting edge within the work is arranged to be an ellipse of which minor axis is oriented in the radial direction connecting a deepest point thereof and the center of the work as viewing in a plane view, and the work is cut as lowering the cutting tool until reaching groove depth of a machining target while maintaining the inclination angle, so that deviation occurring in a direction of an axis toward a work center axis in parallel to a work face from a point on the work face on which the deepest point is projected or an axis of the rotating shaft of the cutting tool projected on the work face can be arbitrarily set.