  One of the mechanisms widely considered for driving relativistic jets in
active galactic nuclei, galactic microquasars, and gamma-ray bursts is the
electromagnetic extraction of the rotational energy of a central black hole,
i.e., the Blandford-Znajek process, although the origin of the electromotive
force in this process is still under debate. We study this process as the
steady unipolar induction in the Kerr black hole magnetosphere filled with a
collisionless plasma screening the electric field (the D field) along the
magnetic field (the B field), i.e., D dot B = 0. We extend the formulations and
arguments made by Komissarov, and generally show that the origin of the
electromotive force is ascribed to the ergosphere. It is explicitly shown that
open magnetic field lines penetrating the ergosphere have a region where the D
field is stronger than the B field in the ergosphere, and it keeps driving the
poloidal currents and generating the electromotive force and the outward
Poynting flux. The range of the possible value of the so-called angular
velocity of the magnetic field line Omega_F is deduced for the field lines
threading the equatorial plane in the ergosphere. We briefly discuss the
relation between our conclusion and the ideal magnetohydrodynamic condition.
