A centralized monitoring system for managing maintenance information of an image forming apparatus has a database for registering a device identifier, to identify a device including an image forming apparatus or a local monitoring apparatus that goes between said image forming apparatus and a server system, as master information. The centralized monitoring system obtains the device identifier communicated from the device. In response to the obtaining of the device identifier, the centralized monitoring system responds to the notifying device to communicate the maintenance information indicating an operating status of the image forming apparatus, even if the obtained device identifier is not registered in the database. Then, if it is determined that the obtained device identifier is not registered in the database, the centralized monitoring system determines whether the device identifier is registered in the database or not.