Disclosed are a mobile robot and a controlling method thereof. The mobile robot comprises: a case; and a sensor unit having two or more sending portions and two or more receiving portions arranged on an outer surface of the case separately and alternately. The plurality of sending portions and receiving portions are arranged in an alternating manner, thereby having a directivity. Also, since signals received through the receiving portions are judged based on a reference value, an area unallowable to be detected for an obstacle sensing is minimized, which allows an obstacle to be detected more accurately. When the obstacle corresponds to a side wall on the basis of a moving path of the mobile robot, a distance between the side wall and the mobile robot is calculated based on signals received by the receiving portion closest to the side wall. Accordingly, the mobile robot can move with maintaining a constant distance from the side wall.