AOMORI--Even ninja could use a refresher course, as evidenced by an authentic textbook for the undercover agents that has popped up here detailing such methods as a ball-and-chain weapon, sleep-inducing drugs and self-defense tips.

The book is believed to have been written in the Edo Period (1603-1867) and is an original because its contents include things that have not been described in previously known textbooks about the art of the ninja.

In addition, stains that were apparently made while medicine or weapons were being created are visible on the book.

The textbook is, “a material that suggests that ninja of the Hirosaki domain (the western half of today’s Aomori Prefecture), one of the foremost powers in the country, were likely actually working,” said Shigeto Kiyokawa, a professor at Aomori University’s department of pharmaceutical sciences, who is responsible for the university’s ninja club.

Tetsuya Ueda, a ninja researcher who lives in Kyoto, discovered the book among the collection at the Hirosaki municipal government-run library in the prefecture in late March.

Kiyokawa, who researches ninja of the Hirosaki domain, deciphered what is written in the 12-page textbook along with Toshio Tsuji, a member of an association studying ancient documents.

As a result, it turned out that the textbook describes a weapon called a “genhi,” which is a lead ball attached to a chain, as well as sleep-inducing drugs made of dried bugs or toads.

The book also provides tips for defending oneself from enemies at an inn through blocking the house’s doorway with tatami mats.

In addition, it includes a ninja’s magical method of sharpening the edge of a sword with frog skin.

There is a written line on the textbook that it was donated from Seizo Iwata to a library official in 1909.

Kiyokawa speculates that Iwata’s father is Heikichi Iwata, who participated in the Boshin Civil War (1868-69) and belonged to the unit shared with the Munakata family. The family is believed to have served as instructors to ninja of the Hirosaki domain. Heikichi lived beneath Hirosaki Castle and near the Munakata family’s residence. Based on such factors, the ninja textbook is believed to have been owned by the Munakata family and provided to the Iwata family.

Kiyokawa believes that the book was written to carry on the traditions of the ninja around 1756 when the ninja unit of Hirosaki domain went into hibernation for a period of time.

He plans to announce his research findings at an International Ninja Research Association meeting to be held in Saga Prefecture in September.

“I hope the discovery of the ninja textbook will prompt more discoveries of such books from other families named Munakata, the number of which is high in the prefecture,” Kiyokawa said.