PURPOSE: When grinding the arched corner section between the tubular section and the shoulder section of the work, to minimize the idle grinding amount by moving the grindstone wheel relatively along the arch parallel with the corner section and positioning from the back end of the prior cutting position to the following cutting start position.

CONSTITUTION: When grinding the arched corner section Wc formed at the interface between the tubular section Wa and the flange section Wb by means of a grindstone wheel G, the grindstone wheel G equipped with the perpendicularly crossing edges Ga, Gb and an arched top Gp will move toward the work W by the distance L1 from the center of curvature OP of the top section Gp of the grindstone wheel G to the working face Wc' of the arched corner section Wc. Furthermore said movement is performed while the center OP is properly selecting a plurality of points on the locus along the circumference of the corner section Wc of the work W and sequentially repeating the advance and back. The moving distance L1, the point on the locus, etc. are performed of the decision control by means of the NC machine.