An obstruction-determining apparatus for preventing a mobile robot from being obstructed in a niche and a boundary-estimation method and medium using the obstruction-determining apparatus are disclosed. More particularly, an obstruction-determining apparatus which can determine whether a mobile robot is obstructed in a niche, and enable a mobile robot to easily escape from an obstacle if the mobile robot is determined to be obstructed in the niche, and a boundary-estimation method and medium for estimating the boundaries of an obstacle with a niche using the obstruction-determining apparatus. The obstruction-determining apparatus includes a contact module which collides with an obstacle above the mobile robot, a contact-operating module which rotates or moves linearly as a result of the collision between the contact module and the obstacle, and a sensing module which detects the rotation or the linear movement of the contact-operating module and determines whether the mobile robot is obstructed in a niche.