China's devaluation of the yuan exposes an undefended flank in the Bank of Japan's efforts to jolt its flagging economy out of decades of deflation, which rely heavily on a solid pick-up in overseas demand.

A growing number of Japan's central bankers are privately voicing concern that the problems behind China's currency move will hit Asian demand harder than expected, threatening a Japanese export rebound they hope will stave off the need for another splurge of monetary easing.

Since April 2013 the bank has ploughed ¥170 trillion into a radical quantitative easing program that some, including some policymakers on the bank's board, think has gone far enough — particularly given its questionable returns. Inflation remains barely above zero, for all the firepower thrown at it.