  In the context of the Calabi-Yau string model we discuss the origin of
characteristic pattern of quark-lepton masses and the CKM matrix. The discrete
$R$-symmetry $Z_{2k+1} \times Z_2$ is introduced and the $Z_2$ is assigned to
the $R$-parity. The gauge symmetry at the string scale is broken into the
standard model gauge group at an intermediate energy scale. At energies below
the intermediate scale down-type quarks and also leptons are mixed with
unobserved heavy states, respectively. On the other hand, there are no such
mixings for up-type quarks. The large mixings between light states and heavy
ones cause different mass pattern of leptons and down-type quarks from that of
up-type quarks and yield a nontrivial CKM matrix for quarks but a trivial one
for leptons. The mixing mechanism is illustrated using a two-generation toy
model.
