In an abnormality detection system of a mobile robot, it is configured such that it is self-diagnosed whether the quantity of state is an abnormal value, or whether at least one of the internal sensor, etc., is abnormal and when an abnormality is self-diagnosed, abnormality information is outputted, then the degree of abnormality is discriminated based on the outputted abnormality information and the robot is driven into a stable state in response to the discriminated degree of abnormality. With this, it becomes possible to effectively utilize the abnormality detection result. In addition, since the robot is driven into a stable state in response to the discriminated degree of abnormality, it becomes possible to render the driving appropriate.