  We calculate both the cubic and the quartic self-couplings of the lighter
scalar Higgs boson without assuming the decoupling limit in the
two-Higgs-doublet model (THDM). In some regions of parameter space of the THDM
where the electroweak phase transition is strongly first order, it is possible
that the quartic self-coupling of the lighter scalar Higgs boson might be
deviated by at least 40 % from the standard model prediction.
