Mizuho Financial Group Inc., the nation's third-biggest bank by market value, booked an unexpected fourth straight quarterly loss on bad-loan and credit costs.

The ¥4.5 billion loss in the three months that ended June 30 compares with a ¥133 billion profit a year earlier, and a ¥538 billion loss in the preceding quarter, the Tokyo-based bank said in a statement Friday.

Bad-loan and credit charges surged 16-fold to ¥76 billion from a year earlier as corporate bankruptcies continued to rise in Japan. Mizuho had the lowest core capital ratio of the nation's three megabanks at the end of the previous financial year, according to Macquarie Group Ltd. analyst Ismael Pili.