  In this paper, we show that a rigidly rotating string can extract the
rotational energy from a rotating black hole. We consider Nambu-Goto strings
stationary with respect to a corotating Killing vector with an uniform angular
velocity $\omega$ in the Kerr spacetime. We show that a necessary condition of
the energy-extraction process is that an effective horizon on the string world
sheet, which corresponds to the inner light surface, is inside the ergosphere
of the Kerr black hole and the angular velocity $\omega$ is less than that of
the black hole $\Omega_\mathrm{h}$. Furthermore, we discuss global
configurations of such strings in both of a slow-rotation limit and the
extremal Kerr case.
