Grinding apparatus

A rotation controlling section of a control unit calculates a maximum thicknesswise difference in a wafer by using a noncontact thickness measuring instrument, and if the maximum thicknesswise difference exceeds a tolerance, relatively changes a rotational speed of a chuck table relative to a rotational speed of a spindle. This can shift a rotational speed ratio, which is a ratio between the rotational speed of the spindle and the rotational speed of the chuck table, from an integer. Therefore, the maximum thicknesswise difference can be made small through spark-out processing. As a result, cyclic variations in thickness value of the wafer are reduced, enabling planarization of a ground surface of the wafer.