  We present one possible mechanism for the giant flares of the Soft Gamma-Ray
Repeaters (SGRs) within the framework of magnetar, i.e., superstrongly
magnetized neutron star model, motivated by the positive period increase
associated with the August 27 event from SGR 1900+14. From the second-order
perturbation analysis of the equilibrium of the magnetic polytrope, we find
that there exist different equilibrium states separated by the energy of the
giant flares and the shift in the moment of inertia to cause the period
increase. This suggests that, if we assume that the global reconfiguration of
the internal magnetic field of $H\simg 10^{16}$ G suddenly occurs, the positive
period increase $\Delta P_t/P_t \sim 10^{-4}$ as well as the energy $\simg
10^{44}$ ergs of the giant flares may be explained. The moment of inertia can
increase with a release of energy, because the star shape deformed by the
magnetic field can be prolate rather than oblate. In this mechanism, since the
oscillation of the neutron star will be excited, a pulsation of $\sim$ ms
period in the burst profile and an emission of the gravitational waves are
expected. The gravitational waves could be detected by the planned
interferometers such as LIGO, VIRGO and LCGT.
