  We performed Population III (Pop III) binary evolution by using population
synthesis simulations for seven different models. We found that Pop III
binaries tend to be binary black holes (BBHs) with chirp mass $M_{\rm chirp}
\sim 30~M_{\odot}$ and they can merge at present day due to long merger time.
The merger rate densities of Pop III BBHs at $z=0$ ranges 3.34--21.2 $\rm
/yr/Gpc^3$ which is consistent with the aLIGO/aVIRGO result of 9.7--101 $\rm
/yr/Gpc^3$. These Pop III binaries might contribute to some part of the massive
BBH gravitational wave (GW) sources detected by aLIGO/aVIRGO. We also
calculated the redshift dependence of Pop III BBH mergers. We found that Pop
III low spin BBHs tend to merge at low redshift, while Pop III high spin BBHs
do at high redshift, which can be confirmed by future GW detectors such as ET,
CE, and DECIGO. These detectors can also check the redshift dependence of BBH
merger rate and spin distribution. Our results show that except for one model,
the mean effective spin $\left\langle \chi_{\rm eff} \right\rangle$ at $z=0$
ranges $0.02$--$0.3$ while at $z=10$ it does $0.16$--$0.64$. Therefore, massive
stellar-mass BBH detection by GWs will be a key for the stellar evolution study
in the early universe.
