Abnormal large oscillations with periods of 10 to 60 min sometimes happen in some Japanese large harbors like Nagasaki-Port in Kyushu and Sendai-port in Tohoku with a narrow entrance and long channel. The large harbor oscillations terminate ship loading and unloading works, break mooring ropes of fisher boats or cause overflows of sea water.
　In the present paper, the amplifications and models of harbor oscillations are investigated through field obsevation and numerical simulation.
　Simulation for long period waves like tunamis is applied to the computation for the harbor oscillation.
　The following main conclusions are drawn:
(1) The validity of the numerical simulation has been confirmed by the comparison with the theoretical amplification factors of rectangular and trianglar harbors. The ratios of the simulated amplitude among observation points and simulated current velocities in Nagasaki-port agree with those of the observation.
(2) The numerical simulation indicated that the large harbor oscillations induced in the large harbors like Nagasaki-Port and Sendai-Port are mainly caused by the resonance to a seishe in a continental shelf outside the harbors and that the oscillation in the small harbors like Tsuruga-port and lida-harbor does not dopend
on the natural oscillation in the harbor but mainly on that in the wide sea area outside the harbor.