This paper presents an efficient extraction method of closed loops as primitive symbols of line drawings, and an robust line drawing un-derstanding system incorporating this extrac-tion method. A graph search technique is used for the symbols extraction method to provide efficient model prediction and verification fa-cilities, and quantitative consideration on the computational cost is shown. The line draw-ing understanding system can efficiently ex-tract models from fairly complicated line draw-ings preserving the computational cost fairly small. 