  One-loop contributions to the Higgs potential at finite temperatures are
discussed in the supersymmetric standard model with four Higgs doublet chiral
superfields as well as a pair of charged singlet chiral superfields. The mass
of the lightest Higgs boson $h$ is determined only by the D-term at the
tree-level in this model, while the triple Higgs boson coupling for $hhh$ can
receive a significant radiative correction. The same nondecoupling effect can
also contribute to realize the sufficient first order electroweak phase
transition, which is required for a successful scenario of electroweak
baryogenesis. This model can be a new candidate for a model in which the baryon
asymmetry of the Universe is explained at the electroweak scale. We also
discuss the implication for the measurement of the triple Higgs boson coupling
at the ILC.
