The yield of 30-year bond slipped 1.5 basis points to 1.820 percent to its lowest since June 12, while that on the 20-year debt eased 1 basis point to 1.605 percent, matching a five-week low hit on Tuesday.

"The reason for relatively good auction results for 30-year yesterday was that 30-year became relatively cheap and which prompted end investors who have long duration to purchase the new 30-year in yesterday auction," said Naomi Muguruma, senior fixed-income strategist at Mitsubishi UFJ Morgan Stanley Securities.

"Having said that, we don't expect the 30-year sector will outperform on the yield curve continuously because the core investors in the 30-year sector have minimum yield target which should be the middle of 1.8. They prefer to purchase above 1.9 percent."

On Tuesday, the Ministry of Finance sold 700 billion yen worth of 30-year debt, with a bid-to-cover of 4.09, up from 3.16 in the previous debt sale.

The 10-year JGB futures were unchanged at 144.08, while the yield on 10-year cash bond was also steady at 0.790 percent to match a nine-year low hit on June 4.

The spread between the 10- and 30-year bonds tightened to 103 basis points to its narrowest in a month.

The Bank of Japan is expected to hold off on easing monetary policy when it concludes a two-day meeting on Thursday, as it is convinced that the country's economy is headed for a moderate recovery as strength in domestic demand eases the pain from slowing global growth.