In a grinding device, first, an eddy-current-sensor detects a distance from the eddy-current-sensor to an outer peripheral surface of a belt layer while a tire to be retreaded held by a pair of half rims is caused to rotate. Thus, an eccentricity amount and an eccentricity direction of the tire to be retreaded with respect to a device axial center, corresponding to the phase of the tire to be retreaded, can be respectively determined, so position control signals corresponding to the phase, eccentricity amount and eccentricity direction of the tire to be retreaded can be generated. Next, on the basis of the position control signal, a carriage causes a rasp to be moved in a radial direction with respect to the tire to be retreaded along the radial direction whose center is a device axial center, and the rasp grinds a tread surface of the rotating tire to be retreaded.