  We discuss the scenario with TeV-scale right-handed neutrinos, which are
accessible at future colliders, while holding down tiny seesaw-induced masses
and sizable couplings to the standard-model particles. The signal with
tri-lepton final states and large missing transverse energy is appropriate for
studying collider signatures of the scenario with extra spatial dimensions. We
show that the LHC experiment generally has a potential to discover the signs of
extra dimensions and the origin of small neutrino masses.
