We calculated binary evolutions of first stars (Pop III) and showed that the typical chirp mass of Pop III binary black holes is ∼30Msun with a total mass of ∼60Msun and the maximum mass of Pop III BBH is more massive than the limit mass of the pair instability supernovae. Our result, which predicted gravitational wave events like GW150914 and GW190521, can explain the origin of massive stellar binary black hole mergers. In this talk, I will explain Pop III binary evolutions and the properties of Pop III binary black hole mergers. There is a good chance to check indirectly the existence of Pop III massive stars by gravitational waves.