In 1775 Jun’an, along with Katsuragawa Hoshu, studied medicine and natural science under Carl Peter Thunberg, a Swedish physician who was attached to the Dutch trading office at Nagasaki, and in return helped Thunberg to carry on his studies in botany.
He was interested in the study of plants, particularly herbs, and the development of natural products. Making many new discoveries in these fields, he even produced, joining with Hiraga Gennai, a kind of asbestos cloth called kakampu. In 1770 he succeeded his father as physician and head of the family. At the same time, becoming interested in Western science and medicine, he began the study of Dutch under the physician Yasutomi Kiseki and, when the opportunity presented itself, called on the Dutch merchants who periodically came to Edo to pay their respects to the shogun.
He was also friendly with the head of the Dutch trading office, Isaac Titsingh; after Titsingh returned
to Holland, Jun’an continued to exchange documents and articles related to research with him. In 1778 he became private physician to the lord of Obama and in time received a stipend amounting to 140 koku. He left a number of writings and translations dealing with Western medicine and other subjects.
In 1771 he joined with Maeno Ryotaku, Sugita Gempaku, and others in undertaking a Japanese translation of a Dutch work entitled Ontleedkundige Tofelen (1624), which in turn was a translation of a German work on anatomy by a physician named Johann Adam Kulmus, and after much labor published the completed translation in 1774 under the title Kaitai shinsho. This was the first translation of a Western medical work to appear in Japan and laid the foundation for the introduction and development of Western medical practices.