South Bisan-seto Bridge is a long span suspension bridge whose anchorages are all located in the sea. They are tall and heavy; accordingly, their eigenfrequencies are low and these may become close to those of the towers. The coupling between these structures is very harmful. On the other hand, the mechanical properties of the ground are difficult to know and the precision of the estimation remains relatively poor. It is therefore indispensable to examine the dynamical properties of the anchorages by means of in situ measurement of vibrations. Natural vibrations of an anchorage are observed and its eigenfrequencies identified. It is concluded that the frequencies of the anchorage are considerably high and the reestimated stiffness of the ground agrees well with that which was formerly obtained from the elastic wave velocity data of the neighboring ground.