  We revisit the monophoton plus missing energy signature at $e^+e^-$ colliders
in supersymmetric (SUSY) models where the gravitino is very light. There are
two possible processes which provide the signal: gravitino pair production and
associated gravitino production with a neutralino, leading the monophoton final
state via an additional photon radiation and via the neutralino decay,
respectively. By using the superspace formalism, we construct a model that
allows us to study the parameter space for the both processes. We show that the
signal cross section and the photon spectra provide information on the masses
of the SUSY particles as well as the SUSY breaking scale.
