PURPOSE: To detect the moment when a grindstone is brought into contact with the body to be worked with a high accuracy, by providing a comparing means which outputs a contact signal at the time when the detection value of a rotation number detecting means is reduce more than a preset reference value.

CONSTITUTION: The rotation number of a grindstone shaft 3 is directly detected from the grindstone shaft 3 which rotates by utilizing a displacement detector 5 for example. The reduction of the rotation number of the grindstone shaft 3 at the moment when a grindstone 2 is brought into contact with the body 1 to be worked is detected by comparing this detection value with the reference value set on a setting circuit 7 by a comparing means 8. Consequently, a rotation number detecting means 5 detects correctly and immediately without receiving the effect of a noise the reduction in the rotation number of the grindstone shaft 3 at the contact time and a contact signal can be output by a comparing means 8 at the moment when the grindstone 2 is brought into contact with the body 1 to be worked.