  A slave-boson functional-integral method has been developed for the Hubbard
model with arbitrary, orbital degeneracy $D$. Its saddle-point mean-field
theory is equivalent to the Gutzwiller approximation, as in the case of
single-band Hubbard model. Our theory is applied to the doubly degenerate ($D =
2$) model, and numerical calculations have been performed for this model in the
paramagnetic states. The effect of the exchange interaction on the
metal-insulator (MI) transition is discussed. The critical interaction for the
MI transition is analytically calculated as functions of orbital degeneracy and
electron occupancy.
