  Smallness of neutrino masses can be explained by introducing a tiny vacuum
expectation value of an extra-Higgs doublet which couples to right-handed
neutrinos ($N_R$). This situation is naturally realized in $\nu$-Two Higgs
Doublet Model ($\nu$THDM), where a TeV-scale seesaw mechanism can work well. We
investigate observable phenomenology of $\nu$THDM at LHC and ILC experiments.
Charged Higgs boson ($H^\pm$) in $\nu$THDM is almost originated from the
extra-Higgs doublet and its coupling strength to neutrinos are not small. Then
this model induces rich phenomenology at the LHC, for example, when
$m_{H^\pm}^{} < M_N$, observable charged tracks can be induced from long lived
charged Higgs. On the other hand, when $m_{H^\pm}^{} > M_N$, right-handed
neutrinos can be long-lived, and secondary vertices may be tagged at the LHC.
The $\nu$THDM also predicts observable lepton number violating process at the
ILC.
