A position measurement section of an on-board device measures a position of a vehicle. A travelling position identification section identifies a travelling position of the vehicle on map data based on the measured position of the vehicle and the map data. Then, a position error measurement section computes error information indicating the difference between the measured position of the vehicle and travelling position. Then, a communications section transmits the error information and the position information to a server. A communication section of the server receives the transmitted error information and position information. An error registration section stores the error information in an error database in association with the received position information. Then, for each respective item of position information, a priority level setting section sets a priority level corresponding to the position information based on the error information stored in the error database.