  We derive the deflection angle up to $O(m^2a)$ due to a Kerr gravitational
lens with mass $m$ and specific angular momentum $a$. It is known that at the
linear order in $m$ and $a$ the Kerr lens is observationally equivalent to the
Schwarzschild one because of the invariance under the global translation of the
center of the lens mass. We show, however, nonlinear couplings break the
degeneracy so that the rotational effect becomes in principle separable for
multiple images of a single source. Furthermore, it is distinguishable also for
each image of an extended source and/or a point source in orbital motion. In
practice, the correction at $O(m^2a)$ becomes $O(10^{-10})$ for the
supermassive black hole in our galactic center. Hence, these nonlinear
gravitational lensing effects are too small to detect by near-future
observations.
