The dollar soared above 100 yen for the first time in more than four years today, driven by improved US economic figures and Tokyo's aggressive credit-easing that aims to revive Japan's sluggish economy.
The US dollar rose as high as 101.18 yen, the first time since April 2009 that the greenback has traded above 100 yen.
The move lifted Japanese stocks to their highest level in more than five years.
The weaker yen is boosting Japan's major car and electronics exporters.
The government said the yen's fall signaled that Prime Minister Shinzo Abe's policy mix of increased public spending and aggressive monetary easing, dubbed "Abenomics," was proving successful.
Kick-starting the economy has been Abe's top priority since he took office late last year.
Japan's monetary easing, and expectations it will help reverse persistent deflation, have helped drive the value of the yen down by over 20% against the dollar since October, when it was trading at around 78 yen.
The yen's sustained fall has annoyed some of Japan's trading partners but generally won support from leaders of other big economies eager to see the world's third-biggest economy recover from two decades of stagnation.
Abe has pushed both fiscal and monetary stimulus strategies to help Japan end a long bout of deflation and support domestic demand.
Japanese officials have fought accusations that Tokyo may be manipulating its currency to give its exporters a boost, and so far international financial institutions generally have backed Abe's approach.
Optimism about the US economy also lifted the dollar after several positive indicators were released. The Labor Department said yesterday that unemployment claims fell to the lowest level in more than five years.
And last week, figures showed that the US economy had added 165,000 jobs in April, lowering the unemployment rate fell to 7.5%.
A weaker yen helps Japan's key exporters by boosting overseas earnings when repatriated and by making goods produced within Japan for export more affordable in markets abroad.
However, it raises costs in yen terms of the imported crude oil and natural gas that resource-scarce Japan must rely on to keep its industries humming and power its cities.
The central bank, under its new governor Haruhiko Kuroda, has vowed to double the monetary base through purchases of government bonds to meet a 2% inflation target within the next two years.
By joining the US Federal Reserve and other major central banks in flooding the economy with cash, the Bank of Japan hopes to get corporations and consumers to begin spending more and end a long malaise.
But Abenomics faces risks, too, including the impact of increased public spending on Japan's already enormous national debt and whether higher inflation will also push up interest rates, raising borrowing costs.