1. A data analysis device for detecting a step on the ground based on data of a cloud of points representing a three-dimensional shape of a feature surface in a target space subject to analysis, comprising: subspace setting means for dividing the target space into a plurality of subspaces, and setting the plurality of subspaces respectively as unit spaces to be analyzed; and boundary search means for searching for a boundary formed by the step on a horizontal plane for each of the unit spaces, wherein the boundary search means variously places a candidate line of the boundary on the horizontal plane in the unit space, sets a strip-shaped area having a predetermined width along the candidate line, determines the strip-shaped area in which the points projected on the horizontal plane are accumulated equal to or more than a criteria set in advance and a cloud of the points have a difference in height equal to or more than a step threshold set in advance as a step neighborhood area, and determines a directional line along a distribution of the cloud of points belonging to the step neighborhood area on the horizontal plane as the boundary.