A method of computing an operating position based on a relative distance between a monitor and a movable body correlating data is obtained in advance for determining a one to one correlation between the relative distance and dimensions on the surface of a photographed object of an image. A relative angle is measured between the photographed object and the monitor. The movable body is photographed using a camera of the monitor. The distance and dimensions between reference points on the image are compared with the correlating data coincident with the measured relative angle.