A robot cleaner is provided. The robot cleaner comprises a bumper to buffer a shock caused by a sudden collision with an unexpected obstacle and an unexpected obstacle detecting means and a controller to change a running direction of the robot cleaner to avoid the obstacle. Accordingly, when an unexpected obstacle appears in front of the cleaner and the cleaner collides with the obstacle, the bumper buffers and absorbs the shock, thereby preventing damage to a cleaner body and inner parts. Also, the robot cleaner can avoid the obstacle and, thus, complete a cleaning operation without stopping the cleaning operation.