  We investigate a multi-orbital extension of the periodic Anderson model with
particular emphasis on electron correlations including orbital fluctuations. By
means of a linearized version of the dynamical mean-field theory, we compute
the renormalization factor, the density of states, the spectral gap and the
local correlation functions for a given set of the intra- and inter-orbital
Coulomb interactions as well as the Hund coupling. It is found that when a
certain condition is met for the intra- and inter-orbital interactions for $f$
electrons, orbital fluctuations are enhanced, thereby enlarging the Kondo
insulating gap. This effect is suppressed in the presence of the Hund coupling.
We also clarify how the Kondo insulator is continuously changed to the Mott
insulator when electron correlations among conduction electrons are increased.
