  We study direct searches of additional Higgs bosons in multi-top-quarks
events at the LHC with the collision energy of 14 TeV as well as the
International Linear Collider (ILC) with the collision energy of 1 TeV. As a
benchmark model, we consider two Higgs doublet models with a softly-broken
discrete $Z_2$ symmetry, where the $t\bar t$ decay mode of additional neutral
Higgs bosons can be dominant if their masses are heavy enough. Thus, the
multi-top-quarks events become an important probe of the extended Higgs sector
at future colliders. We estimate the discovery reach at the LHC and the ILC,
and find that the search at the ILC can survey the parameter regions where the
LHC cannot cover.
