  The existence of the ergoregion of the Kerr space-time has not been confirmed
observationally yet. We show that the confirmation would be possible by
observing the quasinormal mode in gravitational waves. As an example, using the
recent population synthesis results of Pop III binary black holes, we find that
the peak of the final merger mass ($M_f$) is about $50~\rm M_{\odot}$, while
the fraction of the final spin $q_f = a_f/M_f > 0.7$ needed for the
confirmation of a part of ergoregion is $\sim 77\%$. To confirm the frequency
of the quasinormal mode, ${\rm SNR} > 35$ is needed. The standard model of Pop
III population synthesis tells us that the event rate for the confirmation of
more than $50\%$ of the ergoregion by the second generation gravitational wave
detectors is $\sim 2.3$ ${\rm events\ yr^{-1}\ (SFR_p/(10^{-2.5}\ M_\odot
yr^{-1}\ Mpc^{-3}))} \cdot (\rm [f_b/(1+f_b)]/0.33)$ where ${\rm SFR_p}$ and
${\rm f_b}$ are the peak value of the Pop III star formation rate and the
fraction of binaries, respectively.
