Context : Aims : We report observation of a large-amplitude filament oscillation followed by an eruption . This is used to probe the pre-eruption condition and the trigger mechanism of solar eruptions . Methods : We used the EUV images from the Extreme-Ultraviolet Imaging Telescope on board SOHO satellite and the H \alpha images from the Flare Monitoring Telescope at Hida Observatory . The observed event is a polar crown filament that erupted on 15 Oct. 2002 . Results : The filament clearly exhibited oscillatory motion in the slow-rising , pre-eruption phase . The amplitude of the oscillation was larger than 20 km s ^ { -1 } , and the motion was predominantly horizontal . The period was about 2 hours and seemed to increase during the oscillation , indicating weakening of restoring force . Conclusions : Even in the slow-rise phase before the eruption , the filament retained equilibrium and behaved as an oscillator , and the equilibrium is stable to nonlinear perturbation . The transition from such nonlinear stability to either instabilities or a loss of equilibrium that leads to the eruption occurred in the Alfvén time scale ( \sim 1 hour ) . This suggests that the onset of the eruption was triggered by a fast magnetic reconnection that stabilized the pre-eruption magnetic configuration , rather than by the slow shearing motion at the photosphere .