The medium-term movement of a longshore bar and the associated cross-shore sediment transport were investigated using beach profile data obtained nearly daily for 8 years from the seaward foot of a dune to a water depth of about 5 m at Hazaki Oceanographical Research Station (HORS). Bar crests were found to move seaward repeatedly with a period of a year. The duration time of the seaward bar migration is shorter than those on the other coasts in the United States, the Netherlands, and New Zealand. Although bars migrated in one direction, seaward, the cross-shore sediment transport rate associated with the bar migrations fluctuated seaward and shoreward. Seaward sediment transport occurred on and around bar crests, whereas shoreward sediment transport occurred in trough regions. The cross-shore sediment transport was active when the offshore wave energy was large, from winter to spring and in autumn. However, the direction of the sediment transport in a region where bars were most developed was different during the two periods; it was seaward from winter to spring and shoreward in autumn. A numerical model for beach profile change confirmed that the development and decay of a bar was caused by the spatial and temporal variations of the cross-shore sediment transport rate.