The Justice Department is investigating Google advertising, and Google may have to pay $500 million, according to a Google filing with the Securities and Exchange Commission on Tuesday. As a result, Google said, last quarter’s net income was 22 percent lower than it originally reported.

The filing did not disclose details and Google declined to comment, saying it was a legal matter. A Justice Department spokesman did not immediately respond to a request for comment.

The filing said:

“In May 2011, in connection with a potential resolution of an investigation by the United States Department of Justice into the use of Google advertising by certain advertisers, we accrued $500 million for the three month period ended March 31, 2011. Although we cannot predict the ultimate outcome of this matter, we believe it will not have a material adverse effect on our business, consolidated financial position, results of operations, or cash flows.”

The investigation has not concluded. Google has set aside $500 million in case of a resolution, and revised its quarterly net income down from $2.3 billion to $1.8 billion. It is unclear whether Google would owe the money to the Justice Department or to advertisers.

Google has been under constant government scrutiny for everything from privacy practices to antitrust concerns, and some of the antitrust inquiries have related to advertising practices. But this investigation is not related to antitrust matters, according to people with knowledge of Google’s dealing with antitrust regulators.