Medaka (Oryzias latipes) populations have recently declinedto such alevel that they are listed as a species in danger of extinction in the Red Data Book. And a decline in the numbers of catfish and loach has been reported in various parts of Japan. One cause of their decline is the increase in the drop height between paddy fields and water channels, particularly drainage channels. For catfish, loach,and medaka, paddy fields are important spawning grounds or habitats. This report describes the results of a study conducted by the construction of an experimental fishway that fish can use to migrate into an idle paddy field beside Lake Kasumigaura and for the observation and determination of the actual migration of fish swimming through the fishway during a 2-year period from 1997 to 1998. It was observed that immediately after the opening of the sluice gate between Lake Kasumigaura and the channel, the number of fishes migrating to the experimental site, which were assumed to have been carried by water flowing from the Lake, reached a peak.