  The Georgi-Machacek (GM) model is one of many Beyond Standard Model scenarios
with an extended scalar sector which can group under the custodial $\rm
SU(2)_C$ symmetry into a fiveplet, a triplet, and two singlets. The heavy
charged custodial fiveplet Higgs $\rm H_5^{\pm}$ are typical particles in GM
model which couple to the electroweak gauge bosons, therefore provide a good
testing ground for the detection of the $\rm H^\pm W^\mp Z$ vertex. The neutral
custodial fiveplet Higgs $\rm H_5^{0}$ in the GM model has the same mass with
$\rm H_5^{\pm}$ and couples to the electroweak gauge bosons $\rm W^+ W^-$ and
ZZ both. We study the discovery prospects of the exotic scalar bosons $\rm
H_5^{\pm}$ and $\rm H_5^{0}$ at the International Linear Collider(ILC) via the
vector boson associated production processes, and discuss two different decay
modes for both charged and neutral scalars. The discovery potential is
discussed. Testing the mass degeneracy of charged and neutral scalar bosons in
the GM model is also considered.
