If we look into the details of each transaction, however, we can see plausible reasons why investors have accepted negative interest rates. Let me briefly mention a few cases where investors would buy T-Bills at negative interest rates. Commercial banks have an incentive to hold a certain amount of liquid short-term assets for the purpose of liquidity and duration risk management.
Among various types of these assets, investors often prefer T-Bills because they are not only eligible collateral for borrowing from the Bank of Japan but also most suitable as collateral pledged against other money market and derivative transactions.