To prevent interruption of route guidance to a destination, when the route guidance by a portable navigation device can be carried by battery driving is used.

During the route guidance, a CPU recognizes the battery residue (S401) and calculates the battery driving time (t) (S404 and S414). This is compared and determined with an estimated duration T to the destination (S405). When this is determined as not being satisfactory, various power saving processes (S430) are executed automatically. As the power saving processes, for example, processes for turning off the output of screen except for the case where it is required for the route guidance, or the like at a branch point and of disabling audio use, aimed at musical piece reproduction or the like are performed.

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