  The NANOGrav collaboration for the pulsar timing array (PTA) observation
recently announced evidence of an isotropic stochastic process, which may be
the first detection of the stochastic gravitational-wave (GW) background. We
discuss the possibility that the signal is caused by the second-order GWs
associated with the formation of solar-mass primordial black holes (PBHs). This
possibility can be tested by future interferometer-type GW observations
targeting the stochastic GWs from merger events of solar-mass PBHs as well as
by updates of PTA observations.
