  Higgs triplet fields are introduced various new physics models such as the
type-II seesaw model, the left-right symmetric model and so on. The vertex of a
charged Higgs boson and weak gauge bosons, $H^\pm W^\mp Z$, appears at the tree
level in these models. The magnitude of this vertex is proportional to the
vacuum expectation value (VEV) of Higgs triplet fields. We discuss the
possibility of measuring this vertex at the ILC and study how precisely
determine the VEV of the triplet field.
