PURPOSE: To measure the diameter of a grinding wheel with high precision without stopping the grinding wheel by measuring the prescribed range and the changing time to be required of the number of rotation of the grinding wheel, and obtaining the diameter of the grinding wheel from the measured time to be required and the torque, the changing range of the number of rotation of the grinding wheel and the width of the grinding wheel.

CONSTITUTION: A rotating grinding wheel 1 is accelerated/decelerated by a motor 4 to be controlled by an inverter device 7 with electrically constant torque, and the number of rotation of the grinding wheel measures the prescribed range and the changing time to be required by a measuring device 6. The diameter of the grinding wheel 1 is obtained by an operation device 8 by this measured time to be required, this torque, the range of change of the number of rotation of the grinding wheel 1, and the width of the grinding wheel 1. As a result, the amount of wear of the grinding wheel, i.e., the condition of the diameter of the grinding wheel can be understood with high precision without completely stopping the rotating grinding wheel body.