In a thread grinder which grinds a thread grove on an inner surface of a workpiece while a rotation axis of a grinding wheel shaft is inclined with respect to an axis of a main shaft, a centering master which has a cylindrical outer surface is mounted to the main shaft. A measuring unit including two measuring instruments such that probes can contact two edges of a grinding wheel is mounted to the centering master. The origins of the measuring instruments are set by the probes contacting the cylindrical outer surface of the centering master. The measuring unit is moved in the axial direction of the centering master to bring the probes of the two measuring instruments into contact with the edges of the grinding wheel, respectively. The position of the grinding wheel shaft is adjusted such that measured values obtained by measuring instruments become same.