1. A lane determining device for determining a travel lane in which a vehicle is traveling, the device comprising: an image information obtaining unit for obtaining image information captured by an imaging device mounted on the vehicle; a vehicle position information obtaining unit for obtaining vehicle position information indicating a current position of the vehicle; a feature information obtaining unit for obtaining, when a road on which the vehicle is traveling has a plurality of lanes, feature information including feature types for target features on respective lanes of the road, in the traveling direction of the vehicle, based on the vehicle position information; an image recognizing unit for image recognition processing of the obtained information to recognize a feature of the type of the target feature on the travel; and a lane accuracy determining unit for determining a vehicle lane accuracy value indicating a level of probability of being the travel lane, for each of the respective lanes of the road on which the vehicle is traveling, wherein the lane accuracy determining unit determines a recognition coefficient for each of the respective lanes, based on the type of the feature recognized as a result of the image recognition processing by the image recognizing unit and the feature type of the target feature of the respective lanes included in the feature information obtained by the feature information obtaining unit, each recognition coefficient indicating a level of probability that the feature type of the target feature has been recognized as the type of the feature recognized by the image recognition processing, and sets, as a vehicle lane accuracy value for each of the respective lanes, a ratio of the recognition coefficient for the lane to the sum of the recognition coefficients of all the lanes of the road on which the vehicle is traveling.