TOKYO, Dec 14 (Reuters) - Super-long Japanese government bonds gained marginally on Thursday after an auction of 1 trillion yen ($8.88 billion) 20-year JGBs saw solid demand, flattening the yield curve.
The 20-year yield dipped 0.5 basis point to 0.560 percent , edging near its November low of 0.550 percent while the 30-year yield ticked down 0.5 basis point to 0.805 percent, its lowest level in about a month.
The 20-year JGB auction produced strong results, with bid-to-cover ratio rising to 4.56 from 4.13 in the previous auction.
The long maturities were supported also because the Bank of Japan, which buys superlong JGBs five times a month, still has four more scheduled purchases in the month.
Other maturities were flat to slightly weaker with the 10-year yield staying flat at 0.045 percent. The five-year yield rose 0.5 basis point to minus 0.115 percent .
The yield spread between five-year and 20-year yields dropped to 67.5 basis points, the smallest level in almost two months.
The price of the benchmark 10-year JGB futures dipped 0.03 point to 150.88.
The U.S. Federal Reserve on Wednesday raised interest rates as expected and kept its rates and economic projections mostly unchanged. Although U.S. bond prices gained following the Fed announcement, that had limited impact on Japanese bonds.
($1 = 112.61 yen) (Reporting by Tokyo Markets Team; Editing by Vyas Mohan)