A segment determination device that is mounted on a vehicle and determines and sets a constant gradient segment in which a gradient can be regarded as constant comprising: 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 (P1-P10) on a road on which a segment where the vehicle travels is defined, from the map unit, wherein the altitude information is map data; a remaining distance calculation unit configured to calculate a distance between a point where the vehicle is located and an end point of the segment among the plurality of points; a reference point determination unit configured to determine a reference point that is a point closest from the end point of the segment among the plurality of points based on the distance calculated by the remaining distance calculation unit; a feature amount determination unit configured to determine a feature amount that indicates gradient information based on altitude information about points including the reference point and a point more ahead of the vehicle than the reference 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 end point of the segment is set as a start point and whichever of two points adjacent to the point ahead of the vehicle 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 a first point ahead of the vehicle and a 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 (P1-P10) on a road on which a segment where the vehicle travels is defined, from the map unit, wherein the altitude information is map data; a remaining distance calculation unit configured to calculate a distance between a point where the vehicle is located and an end point of the segment among the plurality of points; a reference point determination unit configured to determine a reference point that is a point closest from the end point of the segment among the plurality of points based on the distance calculated by the remaining distance calculation unit; a feature amount determination unit configured to determine a feature amount that indicates gradient information based on altitude information about points including the reference point and a point more ahead of the vehicle than the reference 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 end point of the segment is set as a start point and whichever of two points adjacent to the point ahead of the vehicle 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 a first point ahead of the vehicle and a 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.