A diagnostic apparatus for diagnosing a symptom of an anomaly in a device configured to perform an operation for making or breaking a circuit, the device being a switch such as a circuit breaker or a disconnector, the apparatus comprising: an acquisition unit configured to acquire measurement data representing a result of measuring a state of the device; an input unit configured to receive an operation for inputting history information and verification data; a factor probability calculation unit configured to calculate a factor probability for each matter that is a factor in a symptom of anomaly observed in the device, the factor probability being a probability that the matter truly corresponds to a factor for the observed symptom of anomaly; a data storage unit configured to store the inputted verification data, the verification data being data verified for each of symptoms of anomaly that occurs in the device, the verification data representing probabilities that two or more matters that are factors for the symptoms of anomaly truly correspond to the factors for the symptoms of anomaly, respectively; a history storage unit configured to store the inputted history information representing a history of maintenance to which the device has been subjected; and an improvement processing unit configured to determine details of a proposal for improving the observed symptom, based on calculation results of the factor probabilities from the factor probability calculation unit, wherein the factor probability calculation unit is configured to calculate the factor probabilities by correcting the verification data on the symptom observed in the device, based on the measurement data representing the current state of the device and based on a current state of the device determined from the history information, the details of the proposal determined by the improvement processing unit include adjustment items and adjustment rates, the adjustment items being proposed to be adjusted for improving a symptom of anomaly, the adjustment rates representing an adjustment balance for each item in a combination of the items, and the adjustment rates are obtained based on a track record of improving a symptom in the device, wherein the diagnostic apparatus further comprises a display unit configured to display calculation results of the factor probabilities from the factor probability calculation unit and the details of the proposal determined by the improvement processing unit. an acquisition unit configured to acquire measurement data representing a result of measuring a state of the device; an input unit configured to receive an operation for inputting history information and verification data; a factor probability calculation unit configured to calculate a factor probability for each matter that is a factor in a symptom of anomaly observed in the device, the factor probability being a probability that the matter truly corresponds to a factor for the observed symptom of anomaly; a data storage unit configured to store the inputted verification data, the verification data being data verified for each of symptoms of anomaly that occurs in the device, the verification data representing probabilities that two or more matters that are factors for the symptoms of anomaly truly correspond to the factors for the symptoms of anomaly, respectively; a history storage unit configured to store the inputted history information representing a history of maintenance to which the device has been subjected; and an improvement processing unit configured to determine details of a proposal for improving the observed symptom, based on calculation results of the factor probabilities from the factor probability calculation unit, wherein the factor probability calculation unit is configured to calculate the factor probabilities by correcting the verification data on the symptom observed in the device, based on the measurement data representing the current state of the device and based on a current state of the device determined from the history information, the details of the proposal determined by the improvement processing unit include adjustment items and adjustment rates, the adjustment items being proposed to be adjusted for improving a symptom of anomaly, the adjustment rates representing an adjustment balance for each item in a combination of the items, and the adjustment rates are obtained based on a track record of improving a symptom in the device, wherein the diagnostic apparatus further comprises a display unit configured to display calculation results of the factor probabilities from the factor probability calculation unit and the details of the proposal determined by the improvement processing unit.