A numerical controller for a grinding machine, wherein one of reference sizing points is selected according to the contents of a numerical control program. Measured analog data indicating the diameter of a workpiece is received continuously to determine if the size of the machined workpiece coincides with the selected reference sizing point. Upon the coincidence of the size of the machined workpiece with the reference sizing point, a sizing signal is provided, and then machining control is changed in response to the sizing signal. An analog signal provided by a measuring device is sampled at a predetermined interval in a predetermined time. The analog signal is tested to see if the analog signal is in a predetermined range, which is decided based upon temperature variation or vibrations of the grinding machine. And the result of test is displayed to facilitate checking the output characteristics of the measuring device. The analog signal is compared with a predetermined reference value, and then the result of comparison is displayed together with a predetermined reference value.