To positively detect a clamping error and to impede the damage of a cutting tool by fitting a vibration sensor to a nonrotating member on the head side of a spindle head of a machine tool.

A vibration sensor 1 is fitted to the end of a housing 3 which is a nonrotating member on the head side, by a screw 2. A shank 8 of a tool holder 6 is inserted in a tapered hole 15, and in the case of fine chip 14 existing between the tapered hole 15 and the shank 8, eccentricity is generated. When a main spindle 5 is rotated at high speed as it is, vibration is generated. This vibration changes in waveform in relation to a time axis, is discriminated to be abnormal in the case of exceeding a set level and to be normal in the case of being below the set level. The abnormality value is amplified and detected in a display unit 12, and an alarm is generated, or a machine base stop signal is outputted if necessary.