  We performed direct N-body simulations of open clusters with four different
metallicities. To investigate the effective spins of merging binary black holes
(BBHs) originated from these open clusters, we calculated the spin evolution of
Wolf-Rayet (WR) stars with close companion stars (BBH progenitors), taking into
account stellar wind mass loss and tidal spin-up of the WR stars. We found that
BBH progenitors with smaller semi-major axes evolve to merging BBHs with
greater effective spins because of strong tidal forces. In the local Universe,
about 16% of merging BBHs get effective spins larger than 0.1 even if BHs and
their progenitors do not get spin angular momenta other than tidal forces
exerted by their companion stars. If we assume that WR stars have flat and
isotropic distribution of dimensionless spins just after common envelope
phases, the effective spin distribution of merging BBHs is similar to that
inferred from gravitational wave observations with LIGO and Virgo.
