Long period waves with periods of 1～5 minutes sometimes terminate the cargo handling and cause the break of mooring ropes in Japanese several ports. The reason why long period waves with small amplitudes cause such damages is the resonance with natural periods of mooring system (vessel-mooring rope). These long periods waves are mainly induced by the set-down waves bounded in the grouping waves. The termination of cargo handling by long period waves becomes significant in the specified ports like Tomakomai(Hokkai-do), Kashima(Ibaragi prefecture), Noshiro(Akita), Shibushi(Miyazaki), etc. Meanwhile, such damages are not reported in the other major ports in Japan.
　To compare the characteristics of long period waves in the suffered ports with those in the non-suffered ports, the field observation for long period waves are carried out in the two typical ports; one is Noshiro-port where the breaks of mooring ropes of vessels (50,000DWT) sometimes occur and another is Naha-port (Okinawa prefecture) where no accident on cargo handling has been reported. The following main conclusions are derived from the observation;
(1)The long period waves with periods of 1～5min and the harbor oscillation with periods of 30min are observed in the inner part of Noshiro-port.
(2)The long period wave profiles derived theoretically from the set-down wave bounded in the grouping wave. Therefore, the long period waves which cause the cargo handling accident are mainly induced by the offshore grouping waves.
(3)In Naha-port, the amplitudes of long period waves are significant at the offshore area. The long period wave amplitudes in the outer region behind of the offshore breakwaters are almost equal to those at the offshore. They become very small and do not cause any damages to the mooring facilities at the inner part of Naha-port. The appropriate arrangements of the small groins and inner breakwaters are effective to reduce the long period waves.