  A systematic study has been made on the metal-insulator (MI) transition of
the doubly degenerate Hubbard model (DHM) in the paramagnetic ground state, by
using the slave-boson mean-field theory which is equivalent to the Gutzwiller
approximation (GA). For the case of infinite electron-electron interactions, we
obtain the analytic solution, which becomes exact in the limit of infinite
spatial dimension. On the contrary, the finite-interaction case is investigated
by numerical methods with the use of the simple-cubic model with the
nearest-neighbor hopping. The mass-enhancement factor, $Z$, is shown to
increase divergently as one approaches the integer fillings ($N = 1, 2, 3$), at
which the MI transition takes place, $N$ being the total number of electrons.
The calculated $N$ dependence of $Z$ is compared with the observed
specific-heat coefficient, $\gamma$, of $Sr_{1-x}La_xTiO_3$ which is reported
to significantly increase as $x$ approaches unity.
