A device for phasing a threaded grinding stone is phased with respect to a workpiece or a disk dresser prior to the engagement of the threaded grinding stone with the workpiece or with the disk dresser during grinding or dressing. In performing this phasing, it is detected, by means of an AE fluid sensor provided to a grinding stone head which rotatably supports the threaded grinding stone, whether the threaded grinding stone has had contact with the workpiece or the disk dresser. Subsequently, on the basis of the phase of the threaded grinding stone at the time when contact was detected, the threaded grinding stone is positioned in a phase where the aforementioned engagement is feasible.