Navigation system and route search method

A navigation system 100 includes: map data 120 expressing a road map which include zonal network data indicating connections among a plurality of parcels; a minimum cost route calculating means for calculating a minimum cost route achieving a least sum of parcel costs corresponding to parcels present over a range extending from an origin parcel to a destination parcel by referencing the map data and using the zonal network data; an alternative route calculating means for calculating an alternative route by adjusting a parcel cost corresponding to a partial parcel which is part of a first parcel group constituting the minimum cost route; and a route search means 150 for searching for a route extending from the origin to the destination included in the first parcel group and for searching for a route extending from the origin to the destination included in a second parcel group constituting the alternative route.