We calculated the binary evolution of first stars (Population III stars) and showed that the typical chirp mass of Population III binary black holes is approximately (30,M_\odot), corresponding to a total mass of approximately (60,M_\odot). We also found that the maximum mass of Population III binary black holes can exceed the conventional upper mass limit expected from pair-instability supernovae. Our studies, which predicted gravitational-wave events similar to GW150914 and GW190521 before their detections, suggest that Population III stars could be one of the origins of massive stellar binary black hole mergers. In this talk, I will discuss the binary evolution of Population III stars and the properties of their binary black hole mergers. Gravitational-wave observations may provide an indirect way to probe the existence of massive first stars in the early Universe.
