  We construct a class of classical solutions in the Berkovits' open
superstring field theory. The resulting solutions correspond to marginal
boundary deformations in conformal field theory. The vacuum energy vanishes
exactly for the solutions. Investigating the theory expanded around one of the
solutions, we find that it reflects the effect of background Wilson lines. The
solution has a well-defined Fock space expression and it is invariant under
space-time supersymmetry transformation.
