  Ground state properties of multi-orbital Hubbard models are investigated by
the auxiliary field quantum Monte Carlo method. A Monte Carlo technique
generalized to the multi-orbital systems is introduced and examined in detail.
The algorithm contains non-trivial cases where the negative sign problem does
not exist. We investigate one-dimensional systems with doubly degenerate
orbitals by this new technique. Properties of the Mott insulating state are
quantitatively clarified as the strongly correlated insulator, where the charge
gap amplitude is much larger than the spin gap. The insulator-metal transitions
driven by the chemical potential shows a universality class with the
correlation length exponent $\nu=1/2$, which is consistent with the scaling
arguments. Increasing level split between two orbitals drives crossover from
the Mott insulator with high spin state to the band insulator with low spin
state, where the spin gap amplitude increases and becomes closer to the charge
gap. Experimental relevance of our results especially to Haldane materials is
discussed.
