  Magnetars are comparatively young neutron stars with ultra-strong surface
magnetic field in the range $10^{14-16}$ G. The old neutron stars have surface
magnetic field some what less $\sim 10^8$ G which clearly indicates the decay
of field with time. One possible way of magnetic field decay is by ambipolar
diffusion. We describe the general procedure to solve for the ambipolar
velocity inside the star core without any approximation. With a realistic model
of neutron star we determine the ambipolar velocity configuration inside the
neutron star core and hence find the ambipolar decay rate and time scale which
is consistent with the magnetar observations.
