  Recent population synthesis simulations of Pop III stars suggest that the
event rate of coalescence of $\sim 30M_\odot$--$30M_\odot$ binary black holes
can be high enough for the detection by the second generation gravitational
wave detectors. The frequencies of chirp signal as well as quasinormal modes
are near the best sensitivity of these detectors so that it would be possible
to confirm Einstein's general relativity. Using the WKB method, we suggest that
for the typical value of spin parameter $a/M\sim 0.7$ from numerical relativity
results of the coalescence of binary black holes, the strong gravity of the
black hole space-time at around the radius $2M$, which is just $\sim 1.17$
times the event horizon radius, would be confirmed as predicted by general
relativity. The expected event rate with the signal-to-noise ratio $> 35$
needed for the determination of the quasinormal mode frequency with the
meaningful accuracy is $0.17$--$7.2$~${\rm
events~yr^{-1}~(SFR_p/(10^{-2.5}~M_\odot~yr^{-1}~Mpc^{-3}))} \cdot (\rm
[f_b/(1+f_b)]/0.33)$ 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. As
for the possible optical counter part, if the merged black hole of mass $M\sim
60M_\odot$ is in the interstellar matter with $n\sim 100~{\rm cm^{-3}}$ and the
proper motion of black hole is $\sim 1~{\rm km~s^{-1}}$, the luminosity is
$\sim 10^{40}~{\rm erg~ s^{-1}}$ which can be detected up to $\sim 300~{\rm
Mpc}$, for example, by Subaru-HSC and LSST with the limiting magnitude 26.
