A device diagnostic apparatus for a working machine including a body and a work device provided on the body, the device diagnostic apparatus diagnosing at least one of components, as a target device, included in the working machine, the apparatus comprising: data judgment means (101; 101A) for, when device information (121; 121A, 121B) including operating condition information and internal state information is inputted, comparing the operating condition information in the device information with operating condition information stored beforehand to judge whether or not both of the operating condition information agree with each other, and then outputting judgment result information, the operating condition information including external environment information of the target device (40, 41, 48, 58, P3, 75, 76), and operation information of the target device, and the internal state information including operation state information of the target device; and state diagnosis means (103; 103A) 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 beforehand, and then outputting the result of the comparison, characterized in that said state diagnosis means (103, 103A) compares the internal state information in the device information with the internal state information stored beforehand to judge whether or not both of the internal state information agree with each other, and when the judgment result information indicates that both of the internal state information agree with each other, said state diagnosis means (103, 103A) outputs diagnostic result on the basis of maintenance information corresponding to the internal state information stored beforehand, whereas when the judgment result information does not indicate that both of the internal state information agree with each other, said state diagnosis means (103, 103A) outputs diagnostic result indicating that an abnormal state has occurred. data judgment means (101; 101A) for, when device information (121; 121A, 121B) including operating condition information and internal state information is inputted, comparing the operating condition information in the device information with operating condition information stored beforehand to judge whether or not both of the operating condition information agree with each other, and then outputting judgment result information, the operating condition information including external environment information of the target device (40, 41, 48, 58, P3, 75, 76), and operation information of the target device, and the internal state information including operation state information of the target device; and state diagnosis means (103; 103A) 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 beforehand, and then outputting the result of the comparison, characterized in that said state diagnosis means (103, 103A) compares the internal state information in the device information with the internal state information stored beforehand to judge whether or not both of the internal state information agree with each other, and when the judgment result information indicates that both of the internal state information agree with each other, said state diagnosis means (103, 103A) outputs diagnostic result on the basis of maintenance information corresponding to the internal state information stored beforehand, whereas when the judgment result information does not indicate that both of the internal state information agree with each other, said state diagnosis means (103, 103A) outputs diagnostic result indicating that an abnormal state has occurred.