TOKYO  DeNA, a Japanese social game company, said on Tuesday that it would acquire Ngmoco, a Silicon Valley iPhone game developer, for $400 million  one of the largest deals involving an iPhone software company and another sign that Apple’s products are fast becoming the hottest mobile game devices on the market.

The acquisition is also the latest in an overseas spending spree by DeNA, which is little known outside of Japan but aims to be a global rival to the big names in social networking and games, including Facebook and Zynga, which makes the FarmVille games.

“The big tide in social gaming is coming, right now,” Tomoko Namba, the founder and chief executive of DeNA, said in an interview. “We’d like to capture it and quickly become the world’s No. 1 mobile gaming platform.”

“We’re only active in the Japanese market, and we haven’t figured out how to cover the Western market,” she said. “We want to enable developers to go cross-device and to go cross-border. And we need this to happen quickly, in about the next one or two years.”