Google Inc. (Nasdaq: GOOG)’s $400 million acquisition of Admeld Inc. was approved by the U.S. Justice Department after regulators found competition in the online display advertising industry ensures the purchase won’t harm consumers.

The department said today its investigation of the deal found Web publishers use multiple display advertising platforms and move business among them in response to price changes or the quality of ad placements.

“This use of multiple display advertising platforms, commonly called ‘multihoming,’ lessens the risks that the market will tip to a single dominant platform,” the department said in a statement.

The acquisition of AdMeld will strengthen Google’s position in online display advertising, the segment of the Internet ad market that’s outpacing others, according to Emarketer Inc., a New York-based digital research firm.

AdMeld offers technology services to Internet publishers that help them boost revenue by managing display ads from hundreds of sources, including ad networks. Customers of the New York-based company, founded in 2007, include News Corp. (NWSA)’s Fox News and the Weather Channel.

It’s the fourth time since 2007 that that the U.S. government has taken a close look at a Google acquisition to determine if it would stifle competition or drive up prices. Google has gained regulatory approval in each instance. In 2008, though, Google backed out of a proposed partnership with Yahoo Inc. to avoid a legal battle with the Justice Department.

The Justice Department is still reviewing Google’s proposed takeover of cell phone maker Motorola Mobility Holdings Inc. That $12.5 billion deal is the biggest in Google’s 13-year history.

The Federal Trade Commission is in the midst of a broader inquiry into whether Google has been abusing its dominance of Internet search to make it harder for people to find rival services and apply pressure on advertisers to pay higher prices. Google has consistently predicted that investigation will be resolved in its favor.

(Bloomberg and The AP contributed to this report.)