Diagnostic device including a hammering sound signal acquisition unit, a frequency characteristic conversion unit, and an abnormality determination unit. The hammering sound signal acquisition unit acquires an i-th hammering sound signal and a j-th hammering sound signal representing hammering sounds with respect to striking applied to an i-th and a j-th struck position of an inspection target. The frequency characteristic conversion unit respectively converts the i-th hammering sound signal and the j-th hammering sound signal into an i-th frequency characteristic and a j-th frequency characteristic. The abnormality determination unit determines presence or absence of an abnormality in the inspection target on the basis of a matching degree of waveforms around peaks in the i-th frequency characteristic and waveforms around peaks in the j-th frequency characteristic. The diagnostic device can be used with an inspection robot to automatically analyze for loose stator wedges.