An immature Siberian Crane Grus leucogeranus straggled to Niigata, Japan, in late autumn 2016 and remained over winter in an area of rice fields. In this paper the bird's diet and foraging behavior during winter 2016/2017 are described. During daytime, the crane foraged alone, mostly on post-harvest rice fields where it fed almost exclusively on Water Chestnut Eleocharis kuroguwai tubers. Other foods, such as rice grains, earthworms, grasshoppers and fishes, though eaten, were negligible in the bird's diet. The density of water chestnut tubers in the foraging area was estimated to be 3.1±3.0/m² and the biomass 2.0±2.1 g/m². The crane's foraging behavior generally included removing any superficial covering material, digging into and removing the soil, exploring the food in the beak and then consuming it with or without rinsing it in groundwater. During foraging, the frequency of food intake varied from 4.8±2.0/10 min to 41.3±9.5/10 min. These results indicate the great significance of water chestnuts in the diet of the Siberian Crane in winter, although further study is necessary for generalization.