In the meantime, he continued his studies and published his best-known works—first "Nihon Shogyoshi" (History of Japanese commerce) and then "Nihon Kogyoshi" (History of Japanese industry)—in 1898; in recognition of these works, he was awarded the title of Doctor of Literature in 1902, which helped establish his position as a historian.
その間も研究を重ねて、明治31年（1898年）には、代表作となる『日本商業史』・『日本工業史』を相次いで刊行、明治35年（1902年）には、両書によって文学博士を授与されて、歴史学者としての地位を築いた。