In the Segami River flowing into Hitachi Port facing the Pacific Ocean, swell waves with solitons in front often propagate from the river mouth, causing the inundation damages. These damages are closely related to the wave run-up of the long-period waves. Field observation of these long-period waves was carried out to investigate the mechanism of the propagation of long-period waves. Waves with an extraordinary long period of 1, 200s, which was generated as seiche on continental shelf, were observed as well as the long period waves with the period of 100s.