Recent population synthesis simulations of Pop III stars suggest that the event rate of coalescence of similar to 30M(circle dot) - 30M(circle dot) 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 similar to 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 similar to 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 a meaningful accuracy is 0.17 - 7.2 events yr(-1) [(SFRp/(10(-2.5)M(circle dot) yr(-1) Mpc(-3)))center dot([f(b)/(1 + f(b))]/0.33)], where SFRp and f(b) are the peak value of the Pop III star formation rate and the fraction of binaries, respectively. As for the possible optical counterpart, if the merged black hole of mass M similar to 60M(circle dot) is in the interstellar matter with n similar to 100 cm(-3) and the proper motion of the black hole is similar to 1 km s(-1), the luminosity is similar to 10(40) erg s(-1) which can be detected up to similar to 300 Mpc, for example, by Subaru-HSC and LSST with the limiting magnitude 26.
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