Beach profile data obtained every weekday for 15 years from 1987 to 2001 along a 427-meter-long pier were analysed by Complex Empirical Orthogonal Function analysis. The seaward bar migration was represented by the first mode, and the migration frequency and the temporal variation of the bar amplitude were estimated with the complex time coefficient of the first mode. Although the seaward bar migration frequency, which is the reciprocal of the duration time of the bar migration cycle, was relatively constant before 1993 and ranged between 1.3 to 0.8 cycle/year (0.7 to 1.25 year/cycle), it gradually decreased after 1994 and reached 0.1cycle/year (10year/cycle) at the end of 1996. Subsequently, it gradually recovered to the frequency before 1993.The bar amplitude had a similar long period fluctuation. It was large before 1993, but small from 1995 to 1997, and then returned to the pre-1993 value. The bar migration frequency was weakly correlated with the offshore wave energy flux and the bar amplitude. The bar amplitude change frequency also had weak correlations with the parameters. The negative correlation between the bar amplitude and its change frequency indicates that there is a negative feedback system in the bar amplitude.