KOKA, Shiga Prefecture--Like the furtive ninja groups it hosted in feudal times, this city in western Japan still has secrets to spill.

In this latest instance, researchers discovered the first known copy of a legendary ninja training manual, making it an unprecedented find. 

The book on ninja secrets and techniques, especially valuable to aspiring assassins back in the day, was painstakingly written down by hand during the Edo Period (1603-1867). It was found within a batch of old documents in a storehouse, almost as if it had been hiding deliberately.

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The copy of “Kanrinseiyo” (Kazuo Matsumura)

The existence of the book, called “Kanrinseiyo,” had long been a matter of conjecture as “Mansenshukai,” one of Japan’s three famous works on ninja arts, refers to it.

The fact that it had been lying almost in plain sight in the city known as the hometown of ninja only added to the outpouring of astonishment over the discovery.

“The book is very significant for ninja studies because it contains descriptions of practical ninjutsu (ninja arts),” said Michifumi Isoda, a professor of Japanese modern history at the International Research Center for Japanese Studies.

The 40-page book lists 48 ninja arts, including ones not previously known.

* Cane gimmick: It resembles a wooden walking cane but its lower half is made of iron. When unfolded, it becomes a fan and can be used as a shield.

* Fashioning “makibishi” (Japanese version of caltrop): Split old bamboo into thin strips and lash them together either in triangular or square-shaped form to create a throwing device. One corner should be always on top, no matter how you throw it.

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The copy of “Kanrinseiyo” (Kazuo Matsumura)

“The old ninjutsu techniques are written down,” said Yuji Yamada, a professor in the Faculty of Humanities at Mie University and vice president of International Ninja Research Center. “It is possible that other copies of the book exist in Koka and Iga (in Mie Prefecture)."

The two cities are said to be the birthplace of ninja.

“Kanrinseiyo” is believed to be the original source material of “Mansenshukai,” which summarized the main points of the earlier work and dealt with ninja arts that were then practiced.

“Mansenshukai” is designated as a cultural asset of Koka city.

But the original “Kanrinseiyo,” which apparently consists of three volumes, has not been found.

The newly discovered book is a copy of the middle volume.

The colophon of the copy is dated 1748, midway through the Edo Period. That suggests it was written about 70 years after “Mansenshukai” was completed.

A research team called Koka-ryu Ninja Finders examined the find and announced June 19 that the book was indeed an authentic copy of “Kanrinseiyo.”

The team based its assessment on the fact the copy describes ninja arts that “Mansenshukai” references and was found in a storehouse which archived old documents related to Yoshikawa clan, who were skilled in ninja arts. The clan served in the Owari domain, now part of western Aichi Prefecture.

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Michifumi Isoda, left, Takamasa Fukushima, center, and Yuji Yamada show off the copy of “Kanrinseiyo” in Koka, Shiga Prefecture, on June 19. (Kazuo Matsumura)

In fiscal 2015, Koka city authorities formed the research team to promote tourism by publicizing the true nature of ninja. The team headed by Isoda has been researching ninja descendants living in the city.

In April last year, Takamasa Fukushima, a specialist on ninja arts, joined the group.

And in December, he discovered the copy of “Kanrinseiyo” in a storehouse in the Konancho district of Koka, where Yoshikawa clan elders held the influential position of headman. The storehouse was owned by the local community association and stored old documents, mainly dating from the Edo Period concerning the Kazuraki area of Konancho.