PURPOSE: To enable the abnormality of a tool to be detected highly precisely by providing a pressure sensor to detect the pressure of pressure fluid at a thrust receiving surface.

CONSTITUTION: Pressure detected by a pressure sensor 40 through a fluid pipeline 38 shows pressure approximately slightly higher than gauge pressure similarly to the peripheral surface part of a spindle 12 opened to the atmospheric pressure. When a broken-off tool 10 is detected on the basis of the output of the pressure sensor 40, it does not reach a workpiece 50 while a normal tool 10 advances about several mms into the workpiece 50, so that a thrust force neither acts on the spindle 12, nor the output of the pressure sensor 40 rises up. Thus, if a value slightly higher than the output of the pressure sensor 40 before the tool 10 contacts the workpiece 10 is set as a threshold value, the broken-off tool can be surely detected when the tool advances about several mms into the workpiece 50.