  The flavor-changing neutral current (FCNC) problem at tree-level is a very
critical defect of the two Higgs doublet extension of standard model (SM). In
this article, a two-Higgs-doublet model (2HDM) in which such defects do not
exist at all is to be demonstrated. The general pattern of matrix pairs which
can be diagonalized simultaneously by a same unitary transformation is proposed
without extra constraints like symmetries or zeros in M matrices. Only an
assumption of the hermiticity of mass matrices is employed in the derivation.
With this assumption, number of parameters in the mass matrix of a specific
fermion type is reduced from eighteen down to five. Eigenvalues and
eigenvectors are analytically derived and it's surprising that unitary
transformation matrix thus derived depends on only two parameters. It is a very
general and elegant way to solve the tree-level FCNC problem radically and it
includes previous similar models as special cases with specific parameter
values.
