  The string theory predicts the unification of the gauge couplings and
gravity. The minimal supersymmetric Standard Model, however, gives the
unification scale $\sim 2\times 10^{16}$ GeV which is significantly smaller
than the string scale $\sim 5\times 10^{17}$ GeV of the weak coupling heterotic
string theory. We study the unification scale of the non-supersymmetric minimal
Standard Model quantitatively at the two-loop level. We find that the
unification scale should be at most $\sim 4\times 10^{16}$ GeV and the desired
Kac-Moody level of the hyper-charge coupling should be $1.33 \lsim \ky \lsim
1.35$ in the absence of significant string threshold effects.
