A machine tool in which a tool holder having a tool is attached to a main spindle and the main spindle is rotated to conduct machining on a workpiece, comprises: a displacement sensor, the impedance of which is changed according to a distance from the displacement sensor to the tool holder, an AC signal being impressed upon the displacement sensor from an amplifying unit, wherein an abnormality of a state, in which the tool holder is attached to the main spindle, is judged from a signal level appearing in the displacement sensor. The machine tool further comprises: a tuning circuit connected to the displacement sensor, the tuning circuit composing a resonance circuit together with the displacement sensor; and an impedance adjustment circuit for adjusting an internal impedance constant of the tuning circuit.