From 2003-2004, the habitat factors of harvest mice were investigated in Shiga prefecture. This study was conducted in a variety grass fields; we recorded factors such as nest distribution, vegetation, the environmental conditions around the nesting sites, and the use of the land. For two years, we conducted research in a total of 122 areas, and 54 nesting sites were detected. The proportion of mountainous areas included in our study was higher than that of the lakeside areas. Nests were found on flood plains, riverbanks, abandoned paddy fields, rice fields, channel for irrigations, uncultivated lands, and road slopes. Most nests were found on flood plains and in abandoned paddy fields. As compared with the annual Gramineae species, a perennial Gramineae species was the dominant vegetation at the nesting sites. Nests were frequently found at nearly all the sites where nests had been found the previous year. Nests were frequently found in the area which includes a high proportion of forest, river or lakeside, and a low proportion of agricultural land. Fields of perennial grass such as the Japanese pampas grass, located close to the rivers in rural mountainous areas situated between mountains were found to be good habitats for harvest mice.