  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.
