DIAGNOSTIC DEVICE, DIAGNOSTIC SYSTEM, DIAGNOSTIC METHOD, AND DIAGNOSTIC PROGRAM

To prevent incorrect information from being given to a driver regarding the estimation of a degradation state of a secondary battery. A diagnostic device () for diagnosing a battery () provided in an electric vehicle (), includes an estimation unit () that estimates a degradation state of the battery (); a derivation unit () that derives an index value indicating validity of data used for the estimation of the degradation state; and a notification unit () that notifies, when accuracy of an estimated value is determined to be low based on the index value, a cause of a decrease in the accuracy of the estimated value.