  We study the phenomenology of the exotic Higgs bosons in the Georgi-Machacek
model at future electron-positron colliders such as the International Linear
Collider (ILC), assuming the collision energies of 500 GeV and 1 TeV. We show
that the existence of the neutral and singly-charged Higgs bosons in the 5-plet
representation under the custodial $SU(2)_V$ symmetry can be readily identified
by studying various energy and invariant mass distributions of the $W^+W^-Z$
final state. Moreover, their masses can be determined with sufficiently high
precision to test the mass degeneracy, a feature due to the custodial symmetry
of the model. A synergy between such searches at the ILC and the doubly-charged
Higgs search at the LHC will make the 5-plet Higgs boson study more
comprehensive.
