A method and a device for detecting the incorrect chucking of a machine tool capable of easily and surely detecting a chucking error; the method, comprising the steps of measuring, by a sensor (12), the change of a distance (d) between the sensor (12) and the outer peripheral surface of the flange part (2B) of a tool holder (2) installed in a spindle (3) through all around the periphery of the tool holder while rotating the tool holder (2), performing a FFT analysis of the measured data by a CPU (18), extracting a fundamental frequency component, and calculating the amplitude thereof, whereby the eccentric amount of the tool holder (2) can be calculated by using the amplitude of the fundamental frequency component and, when the eccentric amount exceeds a preset allowable value, the tool holder (2) is judged to cause an incorrect chucking.