  We study the ferromagnetism due to orbital degeneracy in the Hubbard model in
infinite dimensions. The model contains the intraorbital repulsion $U$, the
interorbital repulsion $U^\prime$, the exchange $J$ (Hund coupling) and the
pair hopping $J^\prime$, where all of them originate from the on-site Coulomb
interaction. The ground state of the effective one-site problem was obtained by
exact diagonalizations. At the 1/4-filling, we found two insulating phases; one
is a ferromagnetic phase with alternating orbital order and the other is
antiferromagnetic one with uniform orbital order. If electrons are doped into
the 1/4-filling, the ferromagnetic phase still survives and becomes metallic,
while the antiferromagnetic phase disappears. This result indicates that the
double-exchange mechanism is relevant to stabilize metallic ferromagnetism in
infinite dimensions.
