Edo-Period Ninja Tactics Revealed

A newly highlighted historical document is offering rare insight into how ninja once thought about timing, stealth, and the natural world. According to a report by Asahi Shimbun, references to ninja techniques have been discovered in an early Edo-period manuscript written by Mizuya Doi, a resident of Iyo Province (modern-day Ehime Prefecture). The text focuses on the life of his ancestor, Sengoku-period warlord Doi Kiyoyoshi (1546–1629), and includes a remarkable passage describing ninja methods in unexpected detail.

The discovery was confirmed by Professor Yuji Yamada of Mie University, one of Japan’s leading ninja researchers and a specialist in ancient and medieval religious history. Professor Yamada found that the eighth volume of Seiryōki—a multi-volume chronicle running to at least 30 volumes—devotes around three pages to a conversation between Kiyoyoshi and a ninja believed to have come from Tanba Province (present-day Kyoto and Hyōgo). In the passage, the ninja explains when it is easiest to infiltrate a house across the four seasons, offering practical observations rooted in daily life rather than romanticised legend.

According to the text, spring is difficult because warmer weather keeps householders awake late into the night. Summer is more favourable due to lighter clothing and easier movement over moats and stone walls, but comes with risks: mosquitoes may wake the occupants, and shorter nights leave less cover for escape. Autumn presents a different challenge. Attentive householders might notice the sudden silence of insects—a natural warning sign. To counter this, the ninja describes a method called mushiawase, which involves taming insects in advance and making them sing. Professor Yamada speculates these insects were likely crickets, carried in a basket to maintain a constant sound while infiltrating, masking the telltale silence that would otherwise arouse suspicion.

"The period from December 20 to January 15 is especially suitable for infiltration - this reflects a timeless truth: during the busy year-end and New Year period, people’s vigilance tends to relax."

Winter, the ninja explains, is generally poor for movement due to cold weather and heavy clothing, yet opportunities still exist—particularly when the head of the household has been drinking and is fast asleep. The manuscript also notes, without explanation, that the period from December 20 to January 15 is especially suitable for infiltration. Professor Yamada suggests this reflects a timeless truth: during the busy year-end and New Year period, people’s vigilance tends to relax—a warning he notes still applies today.

As reported by Asahi Shimbun, this is the first known written record of mushiawase, and while it remains unclear whether the technique was widely practised, the passage offers compelling evidence of the ingenuity and careful observation that characterised real historical ninja. Far from shadowy myths, these accounts reveal shinobi as pragmatic specialists, closely attuned to seasons, human habits, and even the behaviour of insects in their pursuit of stealth.