A device diagnostic apparatus (201) comprising: data judgment means (101) for, when device information (121) including operating condition information and internal state information is inputted, comparing the operating condition information of the device information with operating condition information stored in a database (111) beforehand to judge whether or not both of the operating condition information agree with each other, and then outputting judgment result information; and state diagnosis means (103) for, when the judgment result information indicates that both of the operating condition information agree with each other, comparing the internal state information in the device information with internal state information stored in the database beforehand, and then outputting the result of the comparison. This makes it possible to reduce the possibility that false judgment result will be output, and to achieve the efficiency of maintenance work.