 "Asian shares steady, yen faces fresh low vs dollar")
Since the BOJ pledged last week to inject about $1.4 trillion into the Japanese economy to end decades of deflation and achieve its target of 2% inflation, the dollar has gained about 6% against the yen to within striking distance of 100 yen, which was last reached in April 2009.
On Thursday, the dollar hit a four-year high of 99.95 yen, the euro climbed as far as 131.10 yen, its highest since January 2010, and the Aussie dollar soared to 105.43 yen, the highest since November 2007.
The dollar was at 99.70 yen and the euro was at 130.72 yen early on Friday.
"Risk sentiment remains buoyant, supported by recent BoJ actions. We expect the positive market momentum to remain in place in the coming days, creating better entry levels for hedging and profit-taking," Barclays Capital said in a research note.
The MSCI's broadest index of Asia-Pacific shares outside Japan was up 0.1%, with Australian shares rising 0.1% and South Korean shares opening up 0.2%.
The Nikkei stock average opened up 0.1% after hitting its highest since July 2008 on Thursday.
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The Nikkei has gained nearly 10% over the past week following the BOJ launched a stimulus campaign of unprecedented scale.
"Unless there are strong catalysts to drive the market higher such as the yen further weaknening to 100 yen against the dollar, profit-taking is natural given the steep rises," said Yutaka Miura, a senior technical analyst at Mizuho Securities.
World equity markets rallied for a fourth day on Thursday, lifted by a surprise drop in the number of Americans seeking unemployment benefits last week, with both the Dow Jones industrial average and the Standard & Poor's 500 Index setting new closing records.
Wall Street rose despite news of a 14% plunge in personal computer sales in the first quarter, the sharpest drop in two decades of record-keeping.
The latest US data eased fears of deterioration in US labor market conditions after a surprise stumble in job growth in March.
Sentiment may be dented by signs of tensions in the Korean peninsula.
A Pentagon spy agency concluded for the first time that North Korea likely has the ability to launch nuclear-armed missiles, illustrating the high stakes surrounding the escalating tensions on the Korean peninsula.
But the Pentagon said on Thursday it would be inaccurate to suggest that North Korea has proven it has the ability to launch a nuclear-armed missile.
US crude futures were steady around $93.48 a barrel.