* Superlong tenor outperforms as yen strengthens
* Buy 20/30 box spread ahead of 20-yaer sale - strategists
By Lisa Twaronite
TOKYO, Jan 23 (Reuters) - Japanese government bonds mostly slipped on Wednesday, giving back some of the gains made in the previous session when the Bank of Japan doubled its inflation target to 2 percent and committed to open-ended asset purchases.
The yield curve flattened as short- and medium-term bonds fell as investors locked in gains, while the recently battered superlong tenor fared better in line with a resurgent yen.
Japan's central bank pledged to extend asset buying into 2014, with no time-limit and an initial plan to buy 13 trillion yen in mostly short-term bills in that year.
"Investors were positioned for an easing announcement, so it's a classic example of 'buy the rumor, sell the fact,'" said Tomohiro Miyasaka, an analyst at Credit Suisse in Tokyo.
Some investors were disappointed that the open-ended buying would not begin until 2014, but the scope of the measures was greater than many had expected.
But JGBs were underpinned by expectations that the BOJ will have to take more easing steps and buy more assets to try to achieve its inflation target.
"There was feeling that the BOJ isn't going to do much in the near-term, but overall, their commitment to easing is supportive for JGBs, particularly short- and medium-term maturities," said a fixed-income fund manager at a Japanese trust bank in Tokyo.
The 10-year JGB yield was flat 0.730 percent, while benchmark 10-year JGB futures slipped 0.08 point to 144.37, moving away from a nearly six-week intraday high of 144.57 hit on Tuesday. Futures remain above their 100-day moving average, now at 144.19.
A stock-market correction limited losses in the bond market, participants said. The Nikkei shed 2.1 percent to a three-week closing low.
The five-year yield added 1.5 basis points to 0.160 after drooping as low as 0.140 percent on Tuesday, the lowest recorded since Japan started issuing 5-year bonds in 2000.
The dollar hit a 2-1/2 year high of 90.25 yen on Monday ahead of the BOJ outcome, but fell back to buy 88.145 yen on Wednesday, bolstering the superlong tenor that sagged in the previous session on fears that the aggressive monetary policy will trigger inflation in the long term, and also on supply concerns ahead of a 20-year sale on Thursday.
The 30-year bond yield fell 2 basis points to 1.950 percent, while the 20-year yield lost 1 point to 1.730 percent.
On Thursday, the Ministry of Finance will offer 1.2 trillion yen of 20-year bonds, either by reopening the number 141 issue, which carries a 1.7 percent coupon, or newly issuing debt with a coupon of 1.8 percent.
Strategists at both RBS Securities and Barclays recommend taking a 20/30 box spread, in which one buys a 20-year asset swap and sells a 30-year asset swap.