  A scenario based on the scale invariance for explaining the vanishing
cosmological constant (CC) is discussed. I begin with a notice on the
miraculous fact of the CC problem that the vacuum energies totally vanish at
each step of hierarchical and successive spontaneous symmetry breakings. I then
argue that the classical scale invariance is a necessary condition for the
calculability of the vacuum energy.
  Next, I discuss how sufficient the scale invariance is for solving the CC
problem. First in the framework of classical field theory, the scale invariance
is shown to give a natural mechanism for realizing the miracle of vanishing
vacuum energies at every step of spontaneous symmetry breakings. Then adopting
Englert-Truffin-Gastmans' prescription to maintain the scale invariance in
quantum field theory, I point out that the quantum scale invariance alone is
not yet sufficient to avoid the superfine tuning of coupling constants for
realizing vanishingly small cosmological constant, whereas the hierarchy
problem may be solved. Another symmetry or a mechanism is still necessary which
protects the flat direction of the potential against the radiative corrections.
