It is reviewed how space-time supersymmetry is realized nonlinearly in open superstring theory without making the GSO projection. We show that the world-sheet string dualities, viz. dualities of open-closed strings and of open-open strings, play crucial roles for the existence of 10 dimensional N=2 supersymmetry in a spontaneously broken phase. We also speculate on a possible mechanism of the restoration of supersymmetry from the viewpoint of world-sheet dynamics.

12 pages, talk presented at the 10th International Symposium on String Theory (July 3-7, Fukuoka, Japan, 2001) and a part of lectures given at the Summer Conference on Strings (July 16-27, Beijing, China, 2001)