The central bank's step underscores the dilemma Tokyo faces in trying to contain unwelcome yen falls, without resorting to interest rate hikes that could derail Japan's fragile recovery.
Register now for FREE unlimited access to Reuters.com Register"Recent rapid and one-sided yen declines are undesirable.
"We will continue to take appropriate steps against excess volatility, while watching currency market developments with a strong sense of urgency," he said.
The government, which holds jurisdiction over currency policy, spent 2.8 trillion yen ($19 billion) in dollar-selling, yen-buying intervention last month when authorities acted in the markets to prop up the yen for the first time since 1998.
The BOJ is widely expected to maintain its massive stimulus programme at its next two-day policy meeting ending Oct. 28.