  It is shown that the complex phase of $B^0$--$\bar {B^0}$ mixing matrix
element is same as that of the standard model one up to the minor correction of
the order of $(m_c/m_t)^2$ or less in the SUSY standard model. This conclusion
is valid as far as the Yukawa coupling constants are perturbative and
realistic, and can be available for other realistic models where generation
mixing and CP violation are solely due to Yukawa interaction among quarks and
Higgs doublets.
