The superlong sector was steady but lagged gains as investors continued to expect pressure on the Bank of Japan for more aggressive easing steps after the Dec 16 election, at which the ruling party is expected to fare poorly.

"Month-end buying will support prices this week, but there are still expectations of more BOJ easing to come, which will weigh on prices at the long end," said a fixed income fund manager at a Japanese trust bank.

"Everyone is waiting for the 10-year auction next week, and those who had hoped to buy at higher yield levels might be disappointed," he added.

The benchmark 10-year JGB futures price ended morning trade up 0.05 point to 144.84, briefly rising to 144.87, their highest level since June 2003, when they hit a record high of 145.09.

The current 10-year cash JGB yield inched down half a basis point to 0.710 percent, its lowest level since June 2003. Benchmark yields fell as low as 0.430 percent that month.

Yields on 20-year bonds were flat at 1.665 percent , with their spread over 10-year yields ticking up to 95.5 basis points, the widest since July 1999.

The BOJ set a 1 percent inflation target in February and has eased monetary policy four times so far this year.

But BOJ board member Sayuri Shirai said on Thursday there was a risk that price growth could undershoot the central bank's forecast, as the overseas slowdown keeps the economy on a weak footing.

Weekly capital flows data released by the Ministry of Finance on Thursday showed that Japanese investors remained net buyers of foreign bonds for the seventh straight week, buying a net 115.3 billion yen in the week through Nov. 24.