The diversion of labour to the completion of the Tōkaidō highway and economical disturbances caused by abortive monetary reforms in 1641 further reduced the margin of agricultural productivity, making famine inevitable.
The eruption of Hokkaido Koma-ga-take in June 1640, resulting in heavy ashfall and plants poisoning in Tsugaru Peninsula and nearby areas, triggered local crop failures which continued into 1642.
Also, the manufacture and sale of non-essential food products, such as millet udon, wheat flour, sōmen, manjū, confections, and soba, was prohibited.
The large displacement of people resulted in wild population fluctuations in Edo and three other major cities of Japan, as crowds searched for the places where starvation was least likely.
The usual stove smoke at dawn and dusk disappeared, people were wandering in gang-like formations, dead bodies were piled on the streets, and infants were abandoned under eaves to starve to death or be devoured by dogs.
Also, together with the expulsion of Christianity from Japan, the Kan'ei Great Famine set a template for how the Bakufu would address country-wide problems, bypassing daimyō.