  In the case of minimal supersymmetric extension of the Standard Model (MSSM),
when the pseudoscalar Higgs boson mass is less than the supersymmetry energy
scale, the effective theory at the electroweak scale is a two-Higgs-doublet
model. We diagonalize the mass matrix of the general two-Higgs-doublet model,
expressing Higgs boson self-couplings in terms of two mixing angles and four
Higgs boson masses, and derive in a compact form the complete set of Feynman
rules, including quartic couplings in the Higgs sector, for the case of
CP-violating potential. Some processes of double and triple Higgs boson
production at a high-energy linear collider are calculated in the case of
mixing angles and scalar boson masses satisfying the MSSM constraints.
