Japan's foreign reserves, the world's second largest after China, hit a record high of $996 billion at the end of January as falling interest rates boosted the value of foreign bonds in the stockpile.
Rising gold prices and the appreciation in the euro also pushed up the value of euro-denominated assets in dollar terms, a ministry official said, with the reserves now 11 percent higher than a year ago.
The yield on the benchmark 10-year U.S. Treasury note fell to 3.597 percent at the end of January, down from 4.027 percent at the end of December, ministry said.
The euro traded at around $1.4861 at the end of January, against around $1.4591 at the end of December.
Japan's reserves ballooned after 20 trillion yen ($188 billion) in yen-selling intervention in 2003 and a further 15 trillion yen in the first three months of 2004, as Tokyo tried to keep a rapid rise in its currency from derailing a fragile economic recovery and accelerating deflation.
Tokyo has kept out of the market since then, but income on the reserves has kept them edging up.
January's reserves figure included $844.5 billion in securities and $124.0 billion in deposits. The rest includes IMF reserve positions, special drawing rights and gold stocks.
Asked about a moderate decline in reserves held in deposits at foreign banks over the past several months, the official said it was due to the ministry's daily reserve management operations, in which it seeks to ensure safely and liquidity before trying to make profits.
Japan does not divulge the currency breakdown of its external reserves but past Japanese currency intervention, which has mostly taken the form of dollar buying, suggests most of Tokyo's foreign cash pile are in the U.S. currency.