In what appears to be the largest investment in the mobile advertising space, InMobi has raised a whooping $200 million from Softbank, a Japanese Internet company, reports GigaOM.

Previously, InMobi raised $15 million from Kleiner Perkins Caufield & Byers and Sherpalo Ventures.

InMobi’s recent funding is expected to help the mobile advertising network compete with Google’s AdMob, Apple’s iAd and independent ad networks such as Millennial Media.

InMobi launched in 2007 and has more than 47 billion mobile ad impressions on its network. In Q2 2011, InMobi served a staggering 2.4 billion impressions in Nigeria.

The mobile advertising space is really growing fast and is expected to reach $24 billion by 2015, according to research firm Informa.