PURPOSE: To previously prevent chatter from being produced in grinding by detecting occurrence of vibration of a whetstone or variations of the RPM of a workpiece in an early time with use of a detector means and thereby issuing an alarm signal.

CONSTITUTION: The RPM VW of a workpiece W is detected by a RPM detector circuit 31, and the detected value is compared with upper limit value NS+α, and lower limit value NS-α in comparators 32a, 32b respectively. A signal is delivered to an OR gate 40 from the comparator 32a when the detected value is over NS+α and from the comparator 32b when it is below NS-α. A display 41 is lighted by an output from the OR gate 40 whereby workers are informed of the possibility of occurrence of chatter. On the other hand, vibration of a whetstone base 25 caused by the vibration of a whetstone G is detected by a vibration detector 30, and the detected signal is compared with a set value in a comparator 38. When the detected value is over the set value, a signal is delivered from the comparator 38 to the OR gate 40 for lighting the display 41.