  The significant heavy threshold effect is found in the supersymmetric SU(5)
model with two adjoint scalars, one of which is interpreted as a massive string
mode decoupled from the lower-energy particle spectra. This threshold related
with the generic mass splitting of the basic adjoint moduli is shown to alter
properly the running of gauge couplings, thus giving a natural solution to the
string-scale grand unification as prescribed at low energies by LEP precision
measurements and minimal particle content. The further symmetry condition of
the (top-bottom) Yukawa and gauge coupling superunification at a string scale
results in the perfectly working predictions for the top and bottom quark
masses in the absence of any large supersymmetric threshold corrections.
