  We argue that primordial gravitational waves have a spectral break and its
information is quite useful for exploring the early universe. Indeed, such a
spectral break can be a fingerprint of the end of inflation, and the amplitude
and the frequency at the break can tell us the energy scale of inflation and
the reheating temperature simultaneously. In order to investigate the spectral
break, we give an analytic formula for evolution of the Hubble parameter around
the end of inflation where the slow roll approximation breaks down. We also
evaluate the spectrum of primordial gravitational waves around the break point
semi-analytically using the analytic formula for the inflation dynamics.
