The grim outlook for smaller regional banks, who are suffering as the rural population shrinks rapidly, is prompting some to dive into a new, potentially lucrative line of business that until now was largely taboo: mergers and acquisitions.

In Japan's traditional banking culture, advising a client to sell a firm was considered unseemly, even rude — implying that the business had failed.

Yet more small business owners in rural areas are struggling to find successors because their children — who have often moved to the big cities — are not interested in taking over. Some of these owners are now being persuaded by the smaller regional banks — which number about 100 — that being acquired isn't such a bad solution.