  We studied the ground and excited states of $^{12}$C based on variational
calculations after spin-parity projection in a framework of antisymmetrized
molecular dynamics(AMD). The calculations systematically reproduce various
experimental data. It was found that the sub-shell closure and SU(3)-limit
$3\alpha$ cluster components are contained in the ground state, while various
$3\alpha$ cluster structures develop in the excited states. We discussed
effects of $\alpha$ breaking and show the importance of coexistence of the
cluster and shell-model-like aspects.
