A navigation system, comprising: map data expressing a road map with a plurality of road links divided into a plurality of parcels, which include zonal network data indicating connections among the plurality of parcels, each determined based upon whether or not a road runs astride successive parcels among the plurality of parcels; an input means for inputting an origin and a destination; a minimum cost route calculating means for calculating a first parcel group constituting a minimum cost route achieving a least sum of travel costs corresponding to parcels present over a range extending from an origin parcel to which the origin belongs to a destination parcel to which the destination belongs, among the plurality of parcels, by referencing the map data and using the zonal network data in the map data; characterized in further comprising an alternative route calculating means for calculating a second parcel group constituting an alternative route extending from the origin parcel to the destination parcel by adjusting a travel cost corresponding to a partial parcel which is part of the first parcel group; and a route search means for searching for a route extending from the origin to the destination, which is made up with the plurality of road links included in the first parcel group and for searching for a route extending from the origin to the destination, which is made up with the plurality of road links included in a second parcel group, wherein the minimum cost route calculated by the minimum cost route calculating means achieves a least sum of travel costs each calculated based upon a distance between the successive parcels, the navigation system further comprising: a high-frequency parcel extracting means for extracting, as the partial parcel, a high-frequency parcel with a passage frequency thereof, with which all routes each connecting parcels among all parcels present within an area over a predetermined range of the road map, pass through the high-frequency parcel, represented by a value greater than a predetermined value, wherein: the alternative route calculating means adjusts the travel cost by increasing the travel costs of traveling the successive parcels that include the partial parcel having been extracted by the high-frequency parcel extracting means. map data expressing a road map with a plurality of road links divided into a plurality of parcels, which include zonal network data indicating connections among the plurality of parcels, each determined based upon whether or not a road runs astride successive parcels among the plurality of parcels; an input means for inputting an origin and a destination; a minimum cost route calculating means for calculating a first parcel group constituting a minimum cost route achieving a least sum of travel costs corresponding to parcels present over a range extending from an origin parcel to which the origin belongs to a destination parcel to which the destination belongs, among the plurality of parcels, by referencing the map data and using the zonal network data in the map data; characterized in further comprising an alternative route calculating means for calculating a second parcel group constituting an alternative route extending from the origin parcel to the destination parcel by adjusting a travel cost corresponding to a partial parcel which is part of the first parcel group; and a route search means for searching for a route extending from the origin to the destination, which is made up with the plurality of road links included in the first parcel group and for searching for a route extending from the origin to the destination, which is made up with the plurality of road links included in a second parcel group, a high-frequency parcel extracting means for extracting, as the partial parcel, a high-frequency parcel with a passage frequency thereof, with which all routes each connecting parcels among all parcels present within an area over a predetermined range of the road map, pass through the high-frequency parcel, represented by a value greater than a predetermined value, wherein: the alternative route calculating means adjusts the travel cost by increasing the travel costs of traveling the successive parcels that include the partial parcel having been extracted by the high-frequency parcel extracting means. the alternative route calculating means adjusts the travel cost by increasing the travel costs of traveling the successive parcels that include the partial parcel having been extracted by the high-frequency parcel extracting means.