  Making use of the extended Ginzburg Landau theory, which includes the fourth
order derivative term, we study the vortex state in a magnetic field parallel
to the $ c$ axis. The vortex core structure is distorted due to this higher
order term, which reveals t he fourfold symmetry. Futher, this distortion gives
rise to the core interaction energy, which favo rs a square lattice tilted by
$45^\circ$ from the $a$ axis. The critical field of this transition is
determined. The magnetization diverges at the transition. This suggests the
transition is of the fi rst order.
