PURPOSE: To shorten the total time associated with a search command when the access time to a disk exceeds plotting time.

CONSTITUTION: This navigation device is so constituted that plural search commands 'a'' are simultaneously outputted and, therefore, the intervals between (read) 'c1' and (search) 'b2' which exist heretofore are narrowed and since the execution of the (search) 'b2' in succession to the (read) 'c1' is possible, the total time associated with the search command when the search time is long with respect to the plotting is shortened. Then, the total time associated with the search command when the access time to the disk exceeds the plotting time is shortened.