Since Carr and Hawking proposed it more than 4 decades ago, primordial black hole (PBH) has attracted people and is increasing its importance more recently. It can explain ubiquitous massive BHs for LIGO/Virgo events, the main component of dark matters, etc., etc. The key point is that, once abundant PBHs are required, it might change the “naturalness” of inflation: the small scale perturbations are free from the CMB scale and might have a large amplitudes. In such a case, stochastic gravitational waves (GWs) induced by the large density perturbations get important. They are not just an evidence of large perturbations but also can carry the information of phase transitions in the universe such as QCD and/or EW. In this talk, I will introduce the astrophysical motivations of PBH, its implication to theories of inflation, and future possibilities in the light of stochastic GWs.