  The doubly-charged scalar boson H^{--} is involved in several new physics
models for generating neutrino masses. Depending on new physics models, H^{--}
has the Yukawa interaction with a pair of left-handed charged leptons or a pair
of right-handed ones. In this talk, we see that these two Yukawa interactions
can be distinguished by measuring energy distributions of charged pions
produced by decays of tau leptons if pair-produced H^{++} H^{--} can decay
sufficiently into four charged leptons which involve one or two tau leptons.
The information on the Yukawa interaction of H^{--} will help to discriminate
models in which neutrino masses are generated.
