Stocks tumbled, the yen strengthened and bond yields melted away after Japan moved to negative interest rates, but the head of the country's giant pension fund is far from fazed.

The Government Pension Investment Fund is still assessing whether rates below zero will be effective in bolstering the economy, and time may be needed to see the positive impact, Norihiro Takahashi said in an interview in Tokyo. Takahashi became president of GPIF this month. It has taken up to 18 months for the benefits to be seen in other regions, and the GPIF has not decided how long it will spend weighing the situation, he said.

The Bank of Japan's surprise announcement of the policy in January sent yields on about 70 percent of the nation's government bonds into negative territory. GPIF had 38 percent of assets in domestic sovereign debt at the end of December, and while the fund has profited from the BOJ-fueled bond rally, the slide in yields makes earning enough money to fund pension payouts a tougher task. Takahashi is in no rush to make changes and says the fund is not advising external managers to avoid bonds with below-zero yields.