We study direct searches of additional Higgs bosons in multi-top-quarks events at the LHC Run-II, its luminosity upgraded version with 3000 fb−1, and the International Linear Collider (ILC) with the collision energy of 1 TeV. Additional Higgs bosons are predicted in all kinds of extended Higgs sectors, and their detection at collider experiments is a clear signature of the physics beyond the standard model. We consider two Higgs doublet models with the discrete symmetry as benchmark models. If these additional Higgs bosons are heavy enough, the decay modes including top quarks can be dominant, and the searches in multi-top-quarks events become an important probe of the Higgs sector. We evaluate the discovery reach in the parameter space of the model, and find that there are parameter regions where the searches at the LHC with 3000 fb−1 cannot survey, but the searches at the ILC 1 TeV run can. The combination of direct searches at the LHC and the ILC is useful to explore extended Higgs sectors.