  We propose a method for confirmation of the existence of Population III (Pop
III) stars with massive black hole binaries as GW150914 in gravitational wave
(GW) observation. When we get enough number of events, we want to determine
which model is closer to reality, with and without Pop III stars. We need to
prepare various "Pop I/II models" and various "Pop I/II/III models" and
investigate which model is consistent with the events. To demonstrate our
analysis, we simulate detections of GW events for some examples of population
synthesis models with and without Pop III stars. We calculate the likelihood
ratio with the realistic number of events and evaluate the probability of
identifying the existence of Pop III stars. In typical cases, our analysis can
distinguish between Pop I/II model and Pop I/II/III model with 90% probability
by 22 GW signals from black hole-black hole binary mergers.
