  We argue that orientable open string theory without GSO projection has N=2
space-time supersymmetry in a spontaneously broken phase. The arguments are
presented both in Neveu-Schwarz-Ramond and Green-Schwarz formulations. The
formal but explicit supersymmetry transformation law for string field is given
in the framework of Witten's open string field theory. Our results support a
fundamental assumption which lies behind the topological construction of stable
D-branes starting from the unstable systems of D9-branes.
