  We discuss a current dynamics on the surface of a 3-dimensional topological
insulator induced by magnetization precession of a ferromagnet attached. It is
found that the magnetization dynamics generates a direct charge current when
the precession axis is within the surface plane. This rectification effect is
due to a quantum anomaly and is topologically protected. The robustness of the
rectification effect against first-varying exchange field and impurities is
confirmed by explicit calculation.
