  We study the effects of the degeneracy of the $e_g$ orbitals as well as the
double exchange interaction with $t_{2g}$ spins in perovskite transition-metal
oxides. In addition to the spin field ${\vec S}_i$, the isospin field ${\vec
T}_i$ is introduced to describe the orbital degrees of freedom. The isospin is
the quantum dynamical variable, and is represented by the boson with a
constraint. The dispersion of this boson is flat along $(\pi/a,\pi/a,k_z)$
($a$: lattice constant) and the other two equivalent directions. This enables
the orbital disordered phase down to low temperatures. We interpret some of the
anomalous experiments, i.e., optical absorption and d.c. resistivity, in the
low temperature ferromagnetic phase of La$_{1-x}$Sr$_x$MnO$_3$ with $x > 0.2$
in terms of this orbital liquid picture.
