A combination of a main navigation apparatus 2 which is fixed to the vehicle and a sub-navigation apparatus 3 which is detachable from the vehicle. When the sub-navigation apparatus 3 is attached, map information and current position information is displayed on a main display 25 under control of a sub-control unit 1, and state of the vehicle is displayed on a sub-display 33. When detaching the sub-navigation apparatus 3 from the vehicle, area map information necessary for waking outside of the vehicle is transmitted from an on-vehicle database 22 to a RAM 32. When the sub-navigation apparatus is detached from the vehicle, if the operation switch 36 is turned on, either the sub-display 33 or a speaker 34 is operated by an sub-navigation unit in a saving power control mode in which either the sub-display 33 or the speaker 34 is operated only for a predetermined period of time since the operation switch 36 is turned on. When information stored in the RAM 32 is not insufficient, additional information is transmitted form the on-vehicle database 22 via a telephone terminal 37.