The yen's third monthly gain against the dollar means the Bank of Japan may decide this week to boost injections of funds into the financial system as Policy Board members seek ways to stem the currency's strength and spur growth.
The BOJ has eased policy at least five times since December 2009, all following yen advances in the prior month. The yen posted a 0.1 percent gain versus the drooping greenback last month and reached a postwar record of 75.95 per dollar on Aug. 19 even as Japan conducted its biggest currency-market intervention in seven years.
The BOJ's key interest rate is the lowest in the world at a range of zero to 0.1 percent, and hasn't been above 0.5 percent since 1995.