Abstract: An articulated arm has a plurality of arms connected to each other by joints. A pointer is connected to one of the plurality of arms. The arm nearest the pointer is the foremost arm of the articulated arm. Angular displacement sensors are provided at joints of the articulated arm, for producing angular signals. Tomographical images of a patient's body are displayed on a cathode-ray tube. The angular signals are converted to tomographical position signals corresponding to the tomographical images on the cathode-ray tube. In response to the tomographical position signals, the position of the tip of the pointer is displayed on the cathode-ray tube with a mark. The system can be used, for example, in locating a position on a patient's body where an operation is to be performed.