NTT DoCoMo lost 40,800 subscribers in November, acknowledging that it's due to customer flight to the iPhone 5, according to Japan's Nikkei business daily.
For Japan's largest mobile-phone carrier, it was the first monthly drop in over five years.
NTT DoCoMo made no bones about blaming its shrinking subscriber base on customers opting for the iPhone 5 at rival carriers.
The company does not offer the iPhone, while rivals Softbank and KDDI do. Both of those carriers posted subscriber gains in November, with KDDI showing net gains of 228,800 customers and Softbank adding 301,900, Nikkei said.
And that's becoming a serious problem for NTT DoCoMo. The iPhone 5 has become the most popular mobile phone in Japan based on monthly sales rankings.
Though the company has avoided the iPhone to date, that may change next year. A NTT DoCoMo company executive said that it has no choice but to cut a deal with Apple, according to Nikkei.