In this paper, we develop the observation device which measures the working radius of a crane. To realize the measurement, the observation device has the CCD camera and the laser range finder, and the specific mark is attached to a crane. Then, we propose methods for the object recognition and the tracking control. As for the tracking control, to realize the control system without time delay, we construct PI control with an extended Kalman filter. It follows from some experiments that the observation device which we developed enables high accuracy measurement of the working radius of a crane