A phasing method for a threaded grinding wheel, in which the threaded grinding wheel is phased with a dresser prior to the engagement of the threaded grinding wheel with the dresser during dressing, the method comprising: rotating the threaded grinding wheel in one direction; detecting, by means of an acoustic emission fluid sensor, elastic waves generated when one of thread surfaces of the threaded grinding wheel comes into contact with a corresponding one of blade surfaces of the dresser; increasing the number of rotations of the dresser until a measured value on one direction side corresponding to the elastic waves exceeds a predetermined value in a case where the measured value on the one direction side does not exceed the predetermined value even if the threaded grinding wheel comes into contact with the dresser; storing a phase on the one direction side of the threaded grinding wheel once the measured value on the one direction side exceeds the predetermined value; rotating the threaded grinding wheel in the other direction; detecting, by means of the acoustic emission fluid sensor, elastic waves generated when the other one of the thread surfaces of the threaded grinding wheel comes into contact with the other one of the blade surfaces of the dresser; increasing the number of rotations of the dresser until a measured value on the other direction side corresponding to the elastic waves exceeds the predetermined value in a case where the measured value on the other direction side does not exceed the predetermined value even if the threaded grinding wheel comes into contact with the dresser; storing a phase on the other direction side of the threaded grinding wheel once the measured value on the other direction side exceeds the predetermined value; and positioning the threaded grinding wheel at a phase where the engagement is feasible, on the basis of the phase on the one direction side and the phase on the other direction side of the threaded grinding wheel. rotating the threaded grinding wheel in one direction; detecting, by means of an acoustic emission fluid sensor, elastic waves generated when one of thread surfaces of the threaded grinding wheel comes into contact with a corresponding one of blade surfaces of the dresser; increasing the number of rotations of the dresser until a measured value on one direction side corresponding to the elastic waves exceeds a predetermined value in a case where the measured value on the one direction side does not exceed the predetermined value even if the threaded grinding wheel comes into contact with the dresser; storing a phase on the one direction side of the threaded grinding wheel once the measured value on the one direction side exceeds the predetermined value; rotating the threaded grinding wheel in the other direction; detecting, by means of the acoustic emission fluid sensor, elastic waves generated when the other one of the thread surfaces of the threaded grinding wheel comes into contact with the other one of the blade surfaces of the dresser; increasing the number of rotations of the dresser until a measured value on the other direction side corresponding to the elastic waves exceeds the predetermined value in a case where the measured value on the other direction side does not exceed the predetermined value even if the threaded grinding wheel comes into contact with the dresser; storing a phase on the other direction side of the threaded grinding wheel once the measured value on the other direction side exceeds the predetermined value; and positioning the threaded grinding wheel at a phase where the engagement is feasible, on the basis of the phase on the one direction side and the phase on the other direction side of the threaded grinding wheel.