Butterfly assemblages were monitored by transect census in Akanuma Experimental Station of Forestry and Forest Products Research Institute, Hatoyama Town, Saitama Prefecture in 2001, 2008 and 2009. Additional collecting and observation were also done in 2001, 2008, 2009, 2010 and 2011, and specimens collected in 1984 were also examined. Sixty-three species of butterflies were recorded and a list of these species is presented with collection data. Twelve of the species are listed in the red data book of Saitama Prefecture. Species richness, abundance and species diversity indices are evaluated based on the transect census. The Sunose environmental index (EI) is also evaluated based on the 63 recorded species and on the transect census. These results indicate a relatively high biodiversity level. Recent changes in the species composition, especially those due to the invasion of some range-expanding species, are discussed.