TOKYO—SoftBank Group sank deeper into the red in the July-September period as it reported write-downs related to the bankruptcy of office-sharing company WeWork and technology shares stayed under pressure amid rising interest rates in the U.S.

The Japanese technology investor posted a net loss of 931.1 billion yen, equivalent to $6.2 billion, for the quarter ended Sept. 30. That is compared with a 477.6 billion yen loss in the April-June quarter and 3.034 trillion yen profit a year earlier, when it unwound its stake in Alibaba Group Holding.