NTT DoCoMo (NYSE: DCM) is losing Takeshi Natsuno, the mastermind behind its hit mobile web service i-Mode, according to AFP, citing a source within the company. Natsuno, 43, is supposed to leave at the end of April. No specifics to where Natsuno will end up next, just that he will be working in the mobile content industry.

Natsuno came to DoCoMo in 1997 from free ISP Hypernet following its flameout; he was initially tasked with trying to figure out how to turn mobile phones into mobile wallets. In 1999, DoCoMo launched i-Mode, the mobile web service that proved a hit with Japanese cell phone users. Two years after its launch, at a time when Europe was trying to overcome WAP backlash, and most US cell phone users were getting their heads around SMS, i-Mode had racked up 25 million users in Japan each paying a $3 monthly subscription charge, plus data charges. DoCoMo was also getting a 9 percent cut in revenues from content providers. Natsuno was feted as i-Mode’s high priest, writing a book about the experience and giving audiences to a revolving door of mobile industry executives and journalists seeking to understand the service’s appeal.

Currently, i-Mode has 48 million subscribers in Japan, though it’s growth has slowed substantially. It hasn’t done as well outside of the country either. Just a week ago, German operator E-Plus said it would shut down i-Mode as of 1 April, becoming one of the last major European operators to kill it off. O2 UK phased out i-Mode last fall, attracting a scant 260,000 users after spending £10 million ($20 million) in licensing and marketing the service. Natsuno once said that European operators didn’t give i-Mode a chance, rolling out flat rate data plans too early.

• Germany’s E-Plus Phasing Out i-Mode