  Primordial black holes (PBHs) are those which may have formed in the early
Universe and affected the subsequent evolution of the Universe through their
Hawking radiation and gravitational field. To constrain the early Universe from
the observational constraint on the abundance of PBHs, it is essential to
determine the formation threshold for primordial cosmological fluctuations,
which are naturally described by cosmological long-wavelength solutions. I will
briefly review our recent analytical and numerical results on the PBH
formation.
