At many ports throughout JAP/4N, port disturbanoes caused by long-period oscilIations are currently posing problems at many harbors and fishing ports but many questions ahout the mechanism ofpQrt disturbances are still remained. in the present studM phenomena were analyzed by field observations and factors were analyzed numerically to clarify the mechanism ofthe port disturbance generation accompanying the advance oflong-period wayes into ports. The results are as fbllows; (1) TIhe port disturbanoes at Kumaishi Fishing Port could be attributed to resonances betwgen the natural periods ofthis ponin Modes 1 through 3 and the long-period components of incoming wayes. (2) The results of observations and nurnerical analyses sho-L ifaport has anatuial beach and water passage, the resonance phenomenon reduces the amplimde amplification facton Kay woivls: Oceanagrqpihic phenomenq Longzperiod oseiZlation, Longzperiodwaves, AlatzLralperibd IVbtural resoncmt mode