Some traders said the market could push on further as investors fret that unrest in the Middle East and North Africa could lift oil prices more and derail global economic growth, although technical charts show the market is near major resistance.
March 10-year futures were up 0.30 point at 139.70 after hitting a three-week high of 139.79.
Futures were extending gains, touching major resistance at 139.79 at the bottom of the cloud on the daily Ichimoku chart. Futures also reached above 75 percent on the RSI to overbought levels.
"All but some Asian shares remained weak, and that prompted buying in JGBs, mainly medium maturities," a trader at a Japanese brokerage said.
"Excessive market reactions to the unrest in the Middle East may end this week, but the situation there may not get any better for at least a few days, so a mood of wariness is likely to remain as tensions could spread to other areas."
In the cash bond market, the benchmark 10-year yield fell 3.0 basis points to 1.245 percent, and the five-year yield was down 2.0 basis points at 0.535 percent, hitting three-week lows.
The 20-year yield dropped 3.5 basis points to 2.020 percent. The 30-year yield fell 5.0 basis points to 2.155 percent, the lowest level in two weeks.
The yield curve flattened as maturities from the long-term outperformed the short-term, with purchases for month-end duration extensions by index-following players supporting bonds with longer maturities.
"Players had been taking risks on a brighter economic outlook, but are now seen continuing to unwind such positions," said Takafumi Yamawaki, chief rates strategist at JP organ Securities in Japan.
JGBs kept gains after Moody's warning the previous day that it may cut Japan's sovereign debt rating. The JGB market has been taking credit rating cuts in stride over the past decade.
The head of Japan's public pension fund (GPIF), the world's largest, said on Wednesday the fund's investment strategy will not be affected by downgrades of Japanese government bonds by credit ratings agencies.
However, he also said that Japan's bulging public debt -- the largest among developed countries at double the size of its economy -- would reach a crucial point in five to 10 years if the problem is not resolved.