PURPOSE: To send a relief robot at the time of failure of a maintenance robot, connecting both robots to each other mechanically by couplers, and electrically by connection plugs, and executing monitoring and maintenance work by the relief robot as a substitute for the maintenance robot to execute the work.

CONSTITUTION: A maintenance robot 4 to run along a running track for performing monitoring and maintenance of each device in a plant is provided with a coupler 23, a connection plug 24, a run articulating mechanism 25, etc. A relief robot 5 is provided with a coupler 33 and a connection plug 35 to be connected to the coupler 23 and the connection plug 24 at the maintenance robot 4 respectively. In addition, a control part 26 having a function of executing monitoring and maintenance functions of the maintenance robot 4 as a substitute, and achieving a diagnosis function, various sensors 28, a relief system changer 29, a pushing rod 34 to push the maintenance robot 4, etc., are provided. At the time of failure of the maintenance robot 4, the relief robot 5 is charged to be connected to the maintenance robot 4 mechanically and electrically for executing its functions as a substitute.