  We study collider signatures for extra scalar bosons in the inert doublet
model at the international linear collider (ILC). The inert doublet model is a
simple extension of the standard model by introducing an additional
isospin-doublet scalar field which is odd under an unbroken Z_2 symmetry. The
model predicts four kinds of Z_2-odd scalar bosons, and the lightest of them
becomes stable and a candidate of the dark matter as long as it is electrically
neutral. Taking into account the constraints from various theoretical and
phenomenological conditions, we perform a simulation study for the distinctive
signatures of the extra scalars over the standard-model background
contributions at the ILC with the center-of-mass energy of sqrt{s} = 250 GeV
and 500 GeV. We further discuss observables for determination of the mass of
the scalars. We find that the parameter regions which cannot be detected at the
large hadron collider can be probed at the ILC.
