To clean all buckets uniformly without consuming a great amount of water in a short time by installing nozzles for spouting water toward the bucket of a bucket line and also equipping a cleaning water supply system for supplying the water to the nozzle.

At the cleaning time of a bucket line 30, during the pause of an unloader or the pausing time of a pier loading unloading, the bucket 32 supplies a cleaning water from a cleaning water supply system 7 to a first nozzle pipe 5 and a second nozzle pipe 6 while circulation running the bucket line 30 slowly in an empty state and the water is discharged from the nozzles to the outer surface of the bucket 32 and a soil and dust adhered to the outer surface are washed out. As the water spouted from plural nozzles installed on the nozzle pipe 5 or 6 arranged on a U shape is discharged from all sides to the outer surface of the bucket 32 and at two places of the upper first nozzle pipe 5 and the lower second nozzle pipe 6, respective buckets 32 can be cleaned surely in a circulating order.