  We study future prospects of the stau which contributes to the Higgs coupling
to di-photon. The coupling is sensitive to new physics and planned to be
measured at percent levels in future colliders. We show that, if the excess of
the coupling is measured to be larger than 4 %, the lightest stau is predicted
to be lighter than about 200 GeV by taking vacuum meta-stability conditions
into account. Such a stau can be discovered at ILC. Moreover, we show how
accurately the stau contribution to the coupling can be reconstructed from the
information that is available at ILC. We also argue that, if the stau mixing
angle is measured, the mass of the heaviest stau can be predicted by measuring
the Higgs coupling, even when the heaviest stau is not yet discovered at the
early stage of ILC.
