The Justice Department has filed its first civil lawsuit under the Ethics in Government Act, against a Wyoming man who ran unsuccessfully for the U.S. Senate in 1984 and refused until last month to file the required financial disclosure statement.

Under law, candidates for Congress are required to report to the Federal Election Commission sources of income, assets and liabilities within 30 days of becoming a candidate.

Stephen Tarver, a retired accountant in Gillette, Wyo., announced in June 1984 that he planned to run in the Sept. 11 Republican primary against Sen. Alan K. Simpson (R-Wyo.). Tarver, who advocated tax reform, received 12 percent of the vote.

He declined yesterday to comment on the suit.

On Jan. 2, 1985, Bonnie St. Clair Parker, staff director of the Senate Select Committee on Ethics, sent Tarver a letter asking for the financial disclosure and providing the proper forms.

Six days later, Tarver replied by letter that the committee was attempting to "invade my privacy."

Tarver complained that the office of the Wyoming secretary of state had failed to inform him of the disclosure requirement. Noting that the notification occurred 3 1/2 months after he was defeated, Tarver said, "a court case might be a very good idea. It would bring to the public's attention that Congress has made it a federal crime to file for U.S. senator punishable by the candidate being required to give up his right to privacy whether he is elected or not."

The Senate ethics panel finally referred the matter to the Justice Department.

Richard K. Willard, who heads the department's Civil Division, sent Tarver a certified letter March 28 informing him that he was subject to the law's disclosure requirements.

"The act does not differentiate between successful and unsuccessful candidates; no invasion of privacy is involved, as you willingly undertook to become a candidate," Willard wrote. He added that Wyoming's secretary of state does not play a role in enforcing federal election laws.

In the letter, Willard warned Tarver that the department would file suit against him April 21 unless Tarver "prior to that date, agreed to submit accurate reports, sign a consent decree and pay an appropriate civil penalty in an amount to be negotiated."

Court documents indicate that Tarver, through an attorney, filed a financial disclosure April 21 but "refused to agree to payment of a civil penalty."

The complaint, filed Tuesday in U.S. District Court in Wyoming, asked the court to declare that Tarver "willfully" violated the ethics act, to impose the maximum allowable fine of $5,000 and to charge him for the government's costs.