  In a class of new physics models, new physics sector is completely or partly
hidden, namely, singlet under the Standard Model (SM) gauge group. Hidden
fields included in such new physics models communicate with the Standard Model
sector through higher dimensional operators. If a cutoff lies in the TeV range,
such hidden fields can be produced at future colliders. We consider a scalar
filed as an example of the hidden fields. Collider phenomenology on this hidden
scalar is similar to that of the SM Higgs boson, but there are several features
quite different from those of the Higgs boson. We investigate productions of
the hidden scalar at the International Linear Collider (ILC) and study the
feasibility of its measurements, in particular, how well the ILC distinguishes
the scalar from the Higgs boson, through realistic Monte Carlo simulations.
