SoftBank Group Corp.'s debt — which is set to swell as its latest global acquisition became effective Monday — is nearing a level at which Moody's Investors Service has said it will consider a rating downgrade.

The Japanese wireless carrier's purchase of British semiconductor designer ARM Holdings PLC for about $32 billion looks set to add to group debt, which at $115 billion on March 31 is five times its adjusted earnings, according to Moody's.

The rating company says any sustained climb above 5.5 times could be a "downward trigger" on its evaluation.