The name, "Shinzaemonnojo NAGAI," which is considered to be Dosan's father's, is also detected in old documents; his name appears in "Todai-ji Temple Joshi Geko Chumon" dated July, 1526 (collected in "The Documents Preserved by Kansei TSUTSUI") and "Bakufu Bugyonin Hoshoan" dated February 19th, 1528 (collected in "Old Documents Gathered by Akita Domain").
父の新左衛門尉と見られる名が古文書からも検出されており、大永6年（1526年）6月付け「東大寺定使下向注文」（『筒井寛聖氏所蔵文書』所収）および大永8年2月19日付「幕府奉行人奉書案」（『秋田藩採集古文書』所収）に「長井新左衛門尉」の名が見えている。