1. A method of building a grid map of a mobile robot, comprising: obtaining grid points by sensing a distance to at least one of boundaries of an external space and an obstacle within boundaries of the external space, and building the grid map; extracting feature points from the grid points; estimating pose of the robot after the robot moves, and updating the feature points using a SLAM (Simultaneous Localization And Map building) algorithm; determining a transform which transforms the feature points extracted from the grid points into the updated feature points; and updating the grid map according to the transform.