Abstract: A tool abnormality detector for use with a machine tool for detecting any abnormality of a tool mounted on the tool mounting shaft. When the tool mounting shaft is in a correct position with respect to the tubular body secured to the spindle, the air pressure is kept constant. When an abnormal resistance is applied to the tool mounting shaft, the latter will lag in rotation with respect to the tubular body, so that the exhaust air passage will be blocked. A pressure sensor detects an abnormal increase in the air supply line, giving a signal for stopping the spindle of the machine tool. This prevents the tool from damage.