The man, identified only by his Twitter account name, Omomo, said his phone was stolen Dec. 29 while riding the Barcelona, Spain, subway, adding he notified his service provider 15 hours later.
His monthly bill came to 994,919 Japanese yen, the overwhelming majority of it run up the day after the loss of the phone, the online newspaper Japan Today reported Friday.
"I feel like my head's sprung a leak," he said in his Twitter message.
He added his contract states the payment of the charges remain his responsibility.
"I talked to the staff at (cellphone service provider) SoftBank shop and the police. I was told that if the thief is caught I won't have to pay the fees, but the chance of that happening is pretty slim," Omomo tweeted.