  We discuss the scenario with gauge singlet fermions (right-handed neutrinos)
accessible at the energy of the Large Hadron Collider. The singlet fermions
generate tiny neutrino masses via the seesaw mechanism and also have sizable
couplings to the standard-model particles. We demonstrate that these two facts,
which are naively not satisfied simultaneously, are reconciled in the
five-dimensional framework in various fashions, which make the seesaw mechanism
observable. The collider signal of tri-lepton final states with transverse
missing energy is investigated for two explicit examples of the observable
seesaw, taking account of three types of neutrino mass spectrum and the
constraint from lepton flavor violation. We find by showing the significance of
signal discovery that the collider experiment has a potential to find signals
of extra dimensions and the origin of small neutrino masses.
