the other common feature of radiative seesaw models, such as the Majorana nature. In the next section, we discuss a possibility of testing the Majorana nature at ILC experiments. At the ILC, properties of the Higgs sector can be measured with much better accuracy than at the LHC, so that we would be able to reconstruct the Higgs potential in any extended Higgs sector if kinematically accessible. Invisible decays of the Higgs boson can also be tested when the branching ratio B ( h → invisible ) is larger than a few % [33]. Furthermore, the Majorana nature in radiative seesaw models; i.e., the existence of TeV scale right-handed Majorana neutrinos or that of lepton number violating interaction, would also be tested at the ILC. In the pair production of charged scalar bosons at the e $^{+}$e − collision, which appear in the radiative seesaw models ( ω $^{+}$ω − in the Zee-Babu model, ξ $^{+}$ξ − in the Ma model, and S $^{+}$S − (and H $^{+}$H $^{−}$) in the AKS model), there are diagrams of the t -channel exchange of left-handed neutrinos or right-handed neutrinos in addition to the usual Drell-Yan type s - channel diagrams. The contribution of these t -channel diagrams is one of the discriminative features of radiative seesaw models, and no such contribution enters into the other extended Higgs models such as the THDM. 5 These t -channel effects show specific dependences on the center-of-mass energy √$_{s}$ in proportion to log s in the production cross section, and enhances the production rates of the signal events for higher values of √$_{s}$$_{. The final states}$ of produced charged scalar boson pairs are quite model dependent but with missing energies; 5 In the minimal supersymmetric standard model (MSSM) selectron pair production can have similar t - channel contributions (the Bino exchange). In such a case, the final state would be something like a e $^{+}$e − pair plus a missing energy. Therefore, we can discriminate it from the radiative seesaw models. 10 A. Electron-positron collisions IV. PHENOMENOLOGY IN RADIATIVE SEESAW MODELS AT THE ILC e $^{+}$e − → ω $^{+}$ω − → ℓ + $_{L}$ℓ − $_{L}$ν$_{L}$ ν$_{L}$ , [The Zee − Babu model] (8) e $^{+}$e − → ξ $^{+}$ξ − → W +( ∗ $^{)}$W − ( ∗ $^{)}$ξ0 $_{r}$ξ 0 r → jjjj ( jjℓ$_{L}$ν$_{L}$ ) ξ 0 $_{r}$ξ 0 r , [The Ma model] (9) e $^{+}$e − → S $^{+}$S − → H $^{+}$H $^{−}$η $^{0}$η0 → τ + $_{R}$τ − $_{R}$ν$_{L}$ ν$_{L}$η $^{0}$η$^{0}$, [The AKS model] (10)