  We study the feasibility of the Type-X two Higgs doublet model (THDM-X) at
collider experiments. In the THDM-X, new Higgs bosons mostly decay into tau
leptons in the wide range of the parameter space. Such scalar bosons are less
constrained by current experimental data, because of the suppressed quark
Yukawa interactions. We discuss a search strategy of the THDM-X with multi-tau
lepton final states at International linear collider and Large Hadron Collider.
By using the collinear approximation, we show that a four tau lepton signature
(e^+e^- -> HA -> 4\tau) can be a clean signal.
