Oscillation of macroscopic variables is discovered in a metastable state of the Hamiltonian system of the mean-field XY model. The duration of the oscillation 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 a self-consistent analysis of the distribution function, as the self-organization of a self-excited swing state through the mean field. The universality of the phenomena is discussed. © 2006 The American Physical Society.