  The Higgs Triplet Model (HTM) is one of important examples for extended Higgs
sectors, because tiny neutrino masses can be simply explained. Unlike the
canonical type-I seesaw model, a scale of new particles can be taken as
$\mathcal{O}(100)$ GeV keeping an enough amount of production cross section for
direct searches at collider experiments. In the HTM, there appear
doubly-charged Higgs bosons $H^{\pm\pm}$, and detection of them is a key to
probe the model. The decay property of $H^{\pm\pm}$ depends on the magnitude of
the vacuum expectation value of the triplet field $v_\Delta$. When $v_\Delta$
is smaller than about 1 MeV, $H^{\pm\pm}$ can mainly decay into the same-sign
dilepton, and the lower mass limit for $H^{\pm\pm}$ had been taken to be about
400 GeV at the LHC. On the other hand, if $v_\Delta$ is larger than about 1
MeV, $H^{\pm\pm}$ can mainly decay into the same-sign diboson. In this case,
the mass bound cannot be applied, so that the scenario based on light
$H^{\pm\pm}$ is still possible. In this talk, we discuss the phenomenology of
the same-sign diboson decay scenario of $H^{\pm\pm}$. First, we review the mass
bound from the current collider experiments given in Ref. \cite{KYY}. We then
discuss the strategy for detection of $H^{\pm\pm}$ at the ILC.
