The bid-to-cover ratio, a gauge of demand, at the 900 billion yen ($8.35 billion) 20-year debt sale rose to 4.25 from 3.58 at the previous auction in April. Analysts said the demand was stronger than expected.
The auction's tail, or gap between the average and lowest accepted prices, tightened to 0.06 from last month's 0.11.
Benchmark 10-year JGB futures barely moved at 152.27, with a trading volume of 9,443 lots, and the 10-year JGB yield was flat at minus 0.005pc.
The 20-year JGB yield fell 1.5 basis points to 0.330pc. The 30-year JGB yield was down 2 basis points to 0.455pc.
At the short end of the market, the two-year JGB yield and the five-year yield stood flat at minus 0.165pc and minus 0.125pc, respectively.