  The superradiant scattering of Alfv\'en waves (Alfv\'enic superradiance) in a
forcefree magnetosphere is discussed to reveal the relationship between the
Blandford-Znajek (BZ) process and superradiance. For simplicity, we consider a
four-dimensional rotating black string spacetime of which each $z=\text{const}$
slice is a Ba\~nados-Teitelboim-Zanelli solution as an analogy of the
equatorial plane of the Kerr spacetime. Then, it is confirmed that the
condition for Alfv\'enic superradiance coincides with that for the BZ process,
and the wave amplification can be very large due to a resonant scattering for
some parameter sets of the wave frequency and the angular velocity of the
magnetic field line. Moreover, by analysis of the Poynting flux, we first show
that the BZ process can be interpreted as the long wavelength limit of
Alfv\'enic superradiance.
