A diagnostic apparatus for an electric drive object that generates a drive force in response to receiving electric power, the diagnostic apparatus includes a circuitry configured to: acquire time series force data associated with the drive force; identify a force oscillation level associated with the drive force based on the force data; set an oscillation threshold at a first level when the drive object operates at a first speed and set the threshold at a second level higher than the first level when the drive object operates at a second speed higher than the first speed; and identify an irregularity of the drive object in response to determining that the force oscillation level exceeds the oscillation threshold.