During 20 years, numbers of Greater White-fronted Geese (Anser albifrons) wintering in Japan have increased 5.6 times (ca. 46,000 in 1999), with over 80% of the geese concentrated around Lake Izunuma-Uchinuma, in northern Japan. Detailed information on the wintering ecology of the geese had not been investigated in Japan and the daily activity pattern and habitat use of the geese were studied around the lake in three winters (October-February), 1996/97, 1997/98 and 1998/99. The daily activity pattern, i.e., flight timing and behavior relating to environmental factors (light intensity, snowfall, temperature and wind speed), was similar to that for several other geese species in temperate areas. The geese wintering in Japan depended exclusively on rice in fields; they foraged in stubble fields, where the density of rice grains was much higher than that in plowed fields. However, the area of stubble fields around the lake has decreased from 1980 to 2000, while combine harvesters replaced machinery, which only cut the rice plants (reapers). The amount of rice grains left in the fields by the combine harvesters was 8.7 times than the reapers. The replaced cropping machinery increased the abundance of food and might facilitate an increase in geese numbers.