  The detection of binary black hole coalescences by LIGO/Virgo has aroused the
interest in primordial black holes (PBHs), because they could be both the
progenitors of these black holes and a compelling candidate of dark matter
(DM). PBHs are formed soon after the enhanced scalar perturbations re-enter
horizon during radiation dominated era, which would inevitably induce
gravitational waves as well. Searching for such scalar induced gravitational
waves (SIGWs) provides an elegant way to probe PBHs. We perform the first
direct search for the signals of SIGWs accompanying the formation of PBHs in
North American Nanohertz Observatory for Gravitational waves (NANOGrav) 11-year
data set. No statistically significant detection has been made, and hence we
place a stringent upper limit on the abundance of PBHs at $95\%$ confidence
level. In particular, less than one part in a million of the total DM mass
could come from PBHs in the mass range of $[2 \times 10^{-3}, 7\times 10^{-1}]
\Msun$.
