Firm results from the Bank of Japan's regular debt-buying operation on nudged JGB yields lower. The central bank bought a total of 710 billion yen ($6.34 billion) of JGBs on Monday.
The benchmark 10-year JGB yield fell 2.5 basis points to 0.010 percent, moving away from a 9-month high of 0.045 percent struck on Friday. The 30-year yield was down 3.5 basis points at 0.570 percent.
Traders said the decline in yields in the superlong JGB maturities such as 30-years was more pronounced due to bids from index-following investors such as life insurers and pension funds, which usually extend the duration of their debt portfolios towards the turn of each month.
The 10-year Treasury note yield fell on Monday to 2.330 percent after rising to a 16-month high of 2.417 percent last week. Tokyo's Nikkei was down 0.4 percent, poised to fall for the first trading day in eight as the yen rebounded against the dollar.