Japan's 40-year government bond auction recorded its strongest demand in six years, as elevated yields drew investors back to the nation's longest-dated debt and offered a potential respite for fragile global markets.

The bid-to-cover ratio reached 3.1 times, the highest since 2020, surpassing the previous auction's 2.82 times and well above the 12-month average of 2.67 times. Demand was partly bolstered by the reduction in 40-year bond issuance in May, while expectations of more aggressive rate hikes from the Bank of Japan also helped ease inflation concerns.

Rinto Maruyama, senior rates and FX strategist at SMBC Nikko Securities, said: "The market expects the Federal Reserve and the Bank of Japan to continue raising rates, which mainly puts upward pressure on short-end yields. At the same time, term premiums are narrowing as the market believes tighter monetary policy will ultimately bring inflation under control."

There is growing speculation that the Bank of Japan could raise rates again as early as next month. The central bank has already decided to lift its benchmark rate to 1.25%, a 31-year high. Kazuo Momma, former head of monetary policy at the Bank of Japan, also endorsed this outlook in an interview.

Meanwhile, Japanese and US policymakers last week used verbal intervention to curb the yen's decline, helping to support market sentiment. The strong auction could bring temporary relief to global bond markets, which have recently been unsettled by factors including rising artificial intelligence spending, inflation triggered by Middle East conflicts, and swelling government debt worldwide. The benchmark 30-year US Treasury yield touched a 22-year high of 5.58% on Monday.

Japan's 40-year bonds are traditionally favored by life insurers for matching long-term liabilities, making this auction a key gauge of whether higher yields are attracting these buyers back. Masayuki Koguchi, chief fund manager at Mitsubishi UFJ Asset Management, said that despite the strong auction result, it still takes time to determine whether underlying demand is truly solid.

Fiscal concerns continue to weigh on Japanese government bonds. Reports earlier suggested that Japan is considering raising its mid-term defense spending target to 3.5% of GDP, aligning the country with NATO and other major US allies. The potential increase has intensified scrutiny of Prime Minister Sanae Takaichi's broader fiscal agenda and how her government would finance additional spending.

Maruyama of SMBC Nikko said that, at the same time, shorter-dated bonds may continue to face pressure amid expectations of accelerated rate hikes by the Bank of Japan.

Market focus has now shifted to Wednesday's two-year bond auction, where investors will closely watch for signals regarding the Bank of Japan's policy path. As one of the maturities most sensitive to rate expectations, the two-year yield is currently hovering around 2%, with traders pricing in the risk of faster monetary tightening.