A diagnostic information management system having a field controller which sends diagnostic information indicating contents of a failure which occurs in the field, to a viewer via a network; and the viewer which displays the diagnostic information sent from the field controller, on a screen. The field controller has a storage section which stores diagnostic information that becomes master diagnostic information of the whole system. The viewer compares the diagnostic information held by itself with the master diagnostic information, and obtains the difference diagnostic information from the field controller side.