A diagnostic device is provided that includes a storage, at least one detector that detects a state of a diagnosis target, and a processor. The processor repeatedly acquires data indicating the state of the diagnosis target from the detector, repeatedly recognizes a rank indicating an abnormality degree of the diagnosis target based on the acquired data, repeatedly stores the acquired data together with the rank and acquisition order information in the storage, and determines whether or not either one of a latest rank indicating the abnormality degree higher than a lowest rank and the latest rank matching the lowest rank in the abnormality degree is established. When the either one is established, the processor stores the latest data in the storage in place of the lowest rank data.