Japanese communications giant Softbank has completed a $200 million investment in San Mateo, Calif.-based mobile advertising network InMobi. The funding will encompass two tranches: Softbank will pony up $100 million this month, delivering the other $100 million in April 2012. Softbank joins existing InMobi investors Kleiner Perkins Caufield & Byers and Sherpalo Ventures, which led an $8 million Series B financing round announced in July 2010.

InMobi will leverage the new investment to build out its mobile advertising ecosystem, also focusing on its SmartPay mobile app billing system as well as HTML5-based rich media production and distribution. Last month, InMobi acquired HTML5 authoring platform Sprout.

The InMobi network now reaches 340 million consumers in more than 165 countries, generating roughly 47 billion mobile ad impressions each month.