  In this paper, we consider the effect of Landau levels on the decay of
superhigh magnetic fields of magnetars. Applying ${}^3P_2$ anisotropic neutron
superfluid theory yield a second-order differential equation for a superhigh
magnetic field $B$ and its evolutionary timescale $t$. The superhigh magnetic
fields may evolve on timescales $\sim (10^{6}-10^{7})$ yrs for common
magnetars. According to our model, the activity of a magnetar may originate
from instability caused by the high electron Fermi energy.
