  We propose a novel method to probe primordial gravitational waves by means of
primordial black holes (PBHs). When the amplitude of primordial tensor
perturbations on comoving scales much smaller than those relevant to Cosmic
Microwave Background is very large, it induces scalar perturbations due to
second-order effects substantially. If the amplitude of resultant scalar
perturbations becomes too large, then PBHs are overproduced to a level that is
inconsistent with a variety of existing observations constraining their
abundance. This leads to upper bounds on the amplitude of initial tensor
perturbations on super-horizon scales. These upper bounds from PBHs are
compared with other existing bounds.
