A security robot that boards a mobile unit to protect the mobile unit from theft is provided. The robot has an internal sensor such as an acceleration sensor installed at the robot and generating an output indicative of condition inside the robot, an external sensor such as CCD cameras installed at the robot and generating an output indicative of condition outside the robot, an abnormality degree discriminator discriminating a degree of abnormality that the mobile unit is experiencing based on information obtained from the outputs of the internal sensor and the external sensor, and an action controller taking preventive action in response to the discriminated degree of abnormality. With this, it becomes possible to discriminate the degree of abnormal situations and act accordingly in response.