AN on-line diagnostic system and method enable equipment information stored in each piece of industrial equipment to be safely disclosed to maintenance personnel to the extent permitted by the user of the industrial equipment. The on-line diagnostics systems comprises industrial equipment and a maintenance apparatus for the maintenance of the industrial equipment, which are connected via the Internet. Equipment information indicating the state of the industrial equipment is encrypted using a specific common key, and the encrypted equipment information is transmitted to the maintenance apparatus in response to a request therefrom. The fact that the common key has been transmitted from the industrial equipment to the maintenance apparatus is outputted. After receiving the encrypted equipment information and the common key that have been transmitted, the encrypted equipment formation is decrypted using the common key, and the decrypted equipment information is outputted.