In the ISAR (Inverse Synthetic Aperture Radar), when a target is to be recognized by use of the radar image produced from the radar echoes, it is important first to estimate the scale of the target. To estimate the scale, the rotating motion of the target must be estimated. This paper describes a method for estimating the scale of the target from the information on the radar image by converting the target figure into a simple model and estimating the rotating motion of the target.