TOKYO, Feb 9 (Reuters) - Japanese government bond prices firmed on Thursday following the Bank of Japan's stepped-up buying in recent sessions and on relief that a 30-year JGB auction was out of the way, even though demand was not particularly strong.
The 10-year JGB yield dipped 0.5 basis point to 0.085 percent, its lowest level since Jan. 31.
After the yield jumped to one-year high of 0.15 percent last week, the BOJ has stepped up buying in the 10-year sector, conducting unlimited buying at 0.110 percent on Friday.
The BOJ has also bought 5-10 year bonds for three times in a row in its bond buying operation since Friday -- a highly unusual move.
Such aggressive buying helped to calm a market rattled by worries that the BOJ may be looking to reduce bond purchases after years of heavy stimulus.
Thursday's auction of 800 billion yen ($7.14 billion) 30-year JGBs drew tepid bids but nonetheless it led to a relief rally in battered superlong bonds.
The 30-year yield fell 3.5 basis points to 0.870 percent , off near one-year peak of 0.915 percent touched on Wednesday.
The auction drew bids of 3.23 times the offer, a tad less than bid-to-cover ratio of 3.33 in the previous auction last month.
The 20-year yield fell 3.5 basis points to 0.680 percent while the 40-year yield fell 4.0 basis points to 1.020 percent.
The yields on superlong bonds such as 20- to 40-year bonds had been rising on a combination of a solid global economic outlook and speculations that the BOJ may tolerate yield rises in these long maturities even under its yield curve control policy.
$1 = 112.12 yen Reporting by Tokyo Markets Team; Editing by Kim Coghill