  The stable existence of a six-$\alpha$ linear structure in highly excited
states of $^{24}$Mg is studied based on a systematic Cranked Hartree-Fock
calculation with various Skyrme-type interactions. Its stability is examined by
allowing the transition of the cluster structure to the shell-model like
structure. Especially, the six-$\alpha$ linear state is exposed to two major
instabilities: the bending motion, which is the main path for the transition to
low-lying states, and the spin-orbit interaction, which is the driving force to
break the $\alpha$ clusters and enhance the independent motion of the nucleons.
The linear structure with large angular momentum is obtained as a meta-stable
stationary state.
