A communication route guidance system provides exchange of data between an information center and each of a plurality of mobile units such as vehicles. The information center has the latest road data and searches it to prepare recommended route guidance data in response to a request from the mobile unit. The reccommended route guidance data is divided into a plurality of data segments, each usable as an independent guidance instruction in the mobile unit. The mobile unit has stored therein road data similar to the road data in the information center and determines if data communication with the information center is available. In a case where the mobile unit can not communicate with the information center, the mobile unit prepares route guidance based on the road data stored in the mobile unit. It thereby becomes possible to guide the user even in the case that the mobile unit can not communicate with the information center. Even in a case where a communication link is disrupted during transmission of route guidance data to the mobile unit, the mobile unit can effectively prepare whole route guidance data in combination with already-received route guidance data and additionally prepared route guidance data by reference to the road data stored in the mobile unit because the already received route guidance data is structured to be usable independently.