This paper describes a method for monitoring orientation of wheel-type or crawler-type robots in a pipe. Orientation of the robot in the pipe is important for controlling locomotion of the robots stably with dead reckoning. Basically, axes connecting left and right wheels are fixed to vehicle body. However our axes are possible to rock around the vehicle body, since the axes are freely connected to the main stem of the vehicle body at front and rear. According as the robot moves in the pipe, rocking angles of the axes are changed, and these angles are collected to know the configuration of the vehicle body. At first the structures of the robot and the free rocking mechanism are explained. Then the method for calculating the orientation utilizing rocking angles is described. Also, the optimum design of the free rocking mechanism is discussed. Finally the prototype of the robot having the monitoring function in the pipe is designed for verifying the monitoring method.