  The anomalous Hall effect in two-dimensional ferromagnets is discussed to be
the physical realization of the parity anomaly in (2+1)D, and the band crossing
points behave as the topological singularity in the Brillouin zone. This
appears as the sharp peaks and the sign changes of the transverse conductance
$\sigma_{xy}$ as a function of the Fermi energy and/or the magnetization. The
relevance to the experiments including the three dimensional systems is also
discussed.
