  We study theoretically the phase diagram of perovskite manganites taking into
account the double degeneracy of the $e_g$ orbitals in a $Mn^{3+}$ ion. A rich
phase diagram is obtained in the mean field theory at zero temperature as
functions of $x$ (hole concentration) and $J_S$ (antiferromagnetic interaction
between $t_{2g}$ spins). The global features of the phase diagram is understood
in terms of the superexchange and double exchange interactions, which are
strongly depends on types of the occupied $e_g$ orbitals. The strong electron
correlation induces the orbital polarization, which controls the dimension of
the conduction band. A sequential change of the spin and orbital structures
with doping holes is consistent with the recent experiments. In particular,
metallic A-type (layered) antiferromagnetic state is found for $x\sim0.5$ with
the uniform $d_{x^2-y^2}$ orbital ordering. Effects of the Jahn-Teller
distortion are also studied.
  A short version of this paper has been already published (to appear in
PRB-rapid June), but this paper contains additional and more detailed results.
