  We discuss first-order electroweak phase transition in models with extended
Higgs sectors for the case with relatively heavy additional scalar bosons. We
first show that, by the combination of the sphaleron decoupling condition,
perturbative unitarity and vacuum stability, mass upper bounds on additional
scalar bosons can be obtained at the TeV scale even at the alignment limit
where the lightest Higgs boson behaves exactly like the SM Higgs boson at tree
level. We then discuss phenomenological impacts of the case with the additional
scalar bosons with the mass near 1 TeV. Even when they are too heavy to be
directly detected at current and future experiments at hadron colliders, the
large deviation in the triple Higgs boson coupling can be a signature for
first-order phase transition due to quantum effects of such heavy additional
Higgs bosons. On the other hand, gravitational waves from the first-order phase
transition are found to be weaker in this case as compared to that with lower
masses of additional scalar bosons.
