There are provided a first navigation unit (transmitting side unit) 10 having a reader 13 for reading at least map data from a map information storage medium 14 and a controller 15 for controlling a system, and a second navigation unit (receiving side unit) 40 having a controller 41 connected to the control unit 15 through a serial communication line 30, buffer memories 42, 46 for storing therein image data from the reader 13, a display image memory 43 for extracting and copying the data in the range required for displaying from the map data developed on the buffer memories 42, 46, and a display controller 44 for displaying the image data on a display 45. The image data (transfer information) with an identification number of a detailed image concerning a rout selecting point approaching just before is stored in the buffer memory 46 with an enough time margin. At a determined display timing, display command information including the identification number is transmitted from the controller 15 to the controller 41 so that the display unit 45 displays the detailed image based on the image data.