To provide a method and a device for detecting looseness of an iron core capable of detecting looseness of a laminated iron plate core constituting an electrical apparatus such as a transformer or a rotary machine separately from mechanical resonant vibration of a coupling member of an apparatus structure.

This device has a constitution equipped with: an exciter 4 provided on an excitation end 2a on the iron core 1 whose looseness is to be detected, for applying vibration having a plurality of frequencies to the iron core 1; vibration sensors 3a, 3b provided on the exciter 4 and a detection end 2b of a portion reached after the vibration is transmitted through the iron core 1; a frequency analysis means 7 for determining a power spectrum classified by the frequency of an excitation end vibration signal and a detection end vibration signal, by receiving the excitation end vibration signal and the detection end vibration signal from the vibration sensors 3a, 3b; an attenuation rate calculation means 9 for calculating the attenuation rate of detection end vibration to attenuation end vibration by using the power spectrum; an attenuation rate change calculation means 10 for calculating the changing rate with time of the attenuation rate; an abnormality determination means 11 for determining whether the iron core 1 has looseness or not from the changing rate with time of the attenuation rate; and a determination result display device 12 for displaying the determination result.