The temporal and spatial behavior of a large-amplitude filament oscillation is investigated using wavelet analysis. The extreme-ultraviolet (EUV) images of the phenomenon, which occurred on 2002 October 15, were taken from the EUV Imaging Telescope on board the Solar and Heliospheric Observatory (SOHO). The wavelet spectra, extracted from the intensity data, show that the filament oscillates as a rigid body, with a period of about 2.5-2.6 hr which is almost constant along the filament. The period slowly decreases with time until the filament erupts. No clear sign of the eruption is found in the wavelet spectrum prior to the eruption, that followed the filament oscillation. The axial component of the magnetic field is estimated between 1 and 5 G, which is believed to be reasonable for a polar crown filament of this kind.