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The dollar rallied to 110.39 yen -- its highest since August 2008 -- in Tokyo after the surprise announcement, up from 109.36 yen in the morning and 109.22 yen in New York Thursday.
The euro rose to 138.44 yen from 137.79 yen in earlier Tokyo trade and 137.75 yen in New York.
The single European currency fetched $1.2591 against $1.2612 in US trade.
The central bank said it would expand the monetary easing scheme following a string of poor data that has fanned fears the economy may contract again in the third quarter.
After a one-day meeting, policymakers said they would add up to 20 trillion yen ($182 billion) to the bank's asset-buying scheme, bringing it to 80 trillion yen annually.