To automatically dress a grinding wheel for grinding the inner surface of a fuel injection nozzle to be formed into a suitable shape.

The tip part of the grinding wheel 8 is cut off to be shaped flat. Since the tip of the grinding wheel 8 is flat, the grinding wheel 8 will not interfere with the tip part of a nozzle seat surface 42. In the case of dressing a slant of the grinding wheel 8, the tip part of the grinding wheel 8 is also dressed to be flat by the same amount, so that the length of a generating line of the slant of the grinding wheel 8 is constant. Thus, the length of the generating line of the slant is kept within a predetermined range, whereby in machining the fuel injection nozzle, the interference between the tip of the grinding wheel 8 and the nozzle seat surface 42 can be prevented.