A segment determination device configured to be mounted on a vehicle and to determine and set a constant gradient segment in which a gradient is regarded as constant the device comprising, when in a state of being mounted on the vehicle a map unit having map data, wherein the map unit is configured to determine a current position of the vehicle; an altitude information acquisition unit configured to acquire altitude information about a plurality of points on a road on which the vehicle travels from the map unit, wherein the altitude information is map data; a feature amount determination unit configured to determine a feature amount that indicates gradient information about the road based on the altitude information about points including a first point ahead of the vehicle in a traveling direction of the vehicle and a second point further ahead of the first point among the plurality of points; and a segment determination unit configured to, in a case where the feature amount is outside of a predetermined range, determine a segment as the constant gradient segment, in which the first point is set as a start point and whichever of two points adjacent to the second point that is closer to the vehicle is set as an end point, wherein the feature amount determination unit determines, as the feature amount, a sum of areas of regions surrounded by a line obtained by connecting the first point ahead of the vehicle and the second point farther from the vehicle than the first point among the plurality of points and a line obtained by sequentially connecting a plurality of points from the first point to the second point among the plurality of points, and wherein, when the vehicle is travelling in the constant gradient segment, the vehicle ensures that a control parameter for controlling a gear stage of the vehicle is constant. a map unit having map data, wherein the map unit is configured to determine a current position of the vehicle; an altitude information acquisition unit configured to acquire altitude information about a plurality of points on a road on which the vehicle travels from the map unit, wherein the altitude information is map data; a feature amount determination unit configured to determine a feature amount that indicates gradient information about the road based on the altitude information about points including a first point ahead of the vehicle in a traveling direction of the vehicle and a second point further ahead of the first point among the plurality of points; and a segment determination unit configured to, in a case where the feature amount is outside of a predetermined range, determine a segment as the constant gradient segment, in which the first point is set as a start point and whichever of two points adjacent to the second point that is closer to the vehicle is set as an end point, wherein the feature amount determination unit determines, as the feature amount, a sum of areas of regions surrounded by a line obtained by connecting the first point ahead of the vehicle and the second point farther from the vehicle than the first point among the plurality of points and a line obtained by sequentially connecting a plurality of points from the first point to the second point among the plurality of points, and wherein, when the vehicle is travelling in the constant gradient segment, the vehicle ensures that a control parameter for controlling a gear stage of the vehicle is constant.