  Oscillation of macroscopic variables is discovered in a metastable state in
the Hamiltonian dynamical system of mean field XY model, the duration of which
is divergent with the system size. This long-lasting periodic or quasiperiodic
collective motion appears through Hopf bifurcation, which is a typical route in
low-dimensional dissipative dynamical systems. The origin of the oscillation is
explained, with self-consistent analysis of the distribution function, as the
emergence of self-excited ``swings'' through the mean-field. The universality
of the phenomena is also discussed.
