Japan Post Bank Co. stands to lose the most from the Bank of Japan's decision to introduce negative interest rates as plummeting bond yields underscore its reliance on the country's debt for income.

The postal bank, one of the biggest holders of Japanese government bonds, gets more than 90 percent of its profit from interest income, according to Shinichiro Nakamura, a Tokyo-based senior analyst at SMBC Nikko Securities Inc. He sees the company's earnings taking a bigger hit from the BOJ's action than the nation's three so-called megabanks, which unlike Japan Post have been expanding abroad and diversifying into fee businesses.

"Domestic yields will deteriorate due to the introduction of negative interest rates," Nakamura said by phone. "And the larger the proportion of domestic interest income in profit, the larger the negative impact."