  The antiferromagnetic ground state of the half-filled Hubbard model with the
doubly degenerate orbital has been studied by using the slave-boson mean-field
theory which was previously proposed by the present author. Numerical
calculations for the simple cubic model have shown that the metal-insulator
transition does not take place except at the vanishing interaction point, in
strong contrast with its paramagnetic solution. The energy gap in the density
of states of the antiferromagnetic insulator is much reduced by the effect of
electron correlation. The exchange interaction $J$ plays an important role in
the antiferromagnetism: although for $J = 0$ the sublattice magnetic moment $m$
in our theory is fairly smaller than $m_{HFA}$ obtained in the Hartree-Fock
approximation, $m$ for $J/U > 0.2$ ($U$: the Coulomb interaction) is increased
to become comparable to $m_{HFA}$. Surprisingly, the antiferromagnetic state is
easily destroyed if a small, negative exchange interaction ($J/U < -0.05$) is
introduced.
