While he was a high-ranking Justice Department official, Supreme Court nominee Douglas H. Ginsburg argued against regulations opposed by the cable television industry and supervised a major departmental investigation into the industry when he had almost $140,000 invested in a cable television company, according to public documents and Justice Department lawyers.

As head of the antitrust division, Ginsburg argued against proposed Federal Communications Commission regulations that would require cable operators to carry certain channels and that would limit the number of cable systems a particular company can own.

He also supervised the antitrust division's investigation into whether cable companies conspired to force pay cable channels, such as Home Box Office, to scramble their signals so that satellite dish owners could not intercept them. The investigation is still technically open, but lawyers familiar with the inquiry said they did not expect action to be taken against the companies.

Ginsburg owned more than 9,000 shares of stock in Rogers Communications Inc., a Toronto-based cable television company, when he was nominated to become a federal appeals court judge here, according to a questionnaire filed with the Senate Judiciary Committee last year.

Ginsburg, who has since sold the stock, apparently held the investment in Rogers when he joined the Justice Department as a deputy assistant attorney general in 1983. At that time, he indicated he would disqualify himself from "all matters" in which the company is "directly affected."

Justice Department spokesman Terry Eastland said, "On both occasions on which he served in the antitrust division, Mr. Ginsburg advised his staff and colleagues in writing that he was recused from all matters involving Rogers Cable Systems, its subsidiaries or affiliates."

The disclosure of Ginsburg's involvement in the cable television investigation and regulations follows reports that as head of the antitrust division he was a "principal participant" in 1986 in the government's efforts to win First Amendment protections for cable television companies.

As first reported by The Associated Press, Ginsburg said he "supervised the drafting" of the government's brief in the case, City of Los Angeles v. Preferred Communications, in which the Supreme Court ultimately agreed with the Justice Department's position.

Eastland said that Ginsburg, "after consultation with staff . . . determined that Rogers would not be affected financially by the Preferred Communications proceeding in the Supreme Court. Rogers was not involved as a party in this or any related proceeding."

White House spokesman Marlin Fitzwater said yesterday that staff attorneys had reviewed Ginsburg's involvement in the Supreme Court case and determined "it's not a problem."

Fitzwater said, "We do not believe it is a conflict of interest or the appearance of a conflict of interest." He said he did not know whether the White House was aware of the situation before Ginsburg was nominated but that "it doesn't make a difference to us because there's no conflict and no appearance of conflict. It's an irrelevant question."

Ethics experts and at least one member of the Senate Judiciary Committee expressed concern about the apparent extent of Ginsburg's involvement in cable matters as a government official at a time when his largest single investment outside real estate was in Rogers.

"Once is a lapse of judgment," said Sen. Patrick J. Leahy (D-Vt.). "If you start doing a lot, then . . . you have to go into questions of sensitivity to following the rules or not. I certainly want to ask questions about it."

Pete Smith, a spokesman for Senate Judiciary Committee Chairman Joseph R. Biden Jr. (D-Del.), said, "We will review it as part of our committee investigation."

Stephen Gillers, a legal ethics expert at New York University School of Law, said Ginsburg's participation in cable matters "raises questions" about his judgment.

"It would be unwise for someone with the sizable investment he had both absolutely and relatively to his total liquid wealth to participate in decisions or in activity the results of which inure to the benefit of the companies in which he has investments," Gillers said.

Ginsburg, who served as assistant attorney general from September 1985 until his nomination to the appeals court a year later, approved Justice Department comments submitted to the Federal Communications Commission on at least two issues of major importance to the cable television industry.

In comments filed with the FCC in January 1986, the department argued against imposing so-called "must carry" requirements on cable operators requiring them to offer all locally available television stations as part of their service. The appeals court here struck down the requirements in a lawsuit brought by a cable company, and the Justice Department opposed the FCC's proposal to impose less stringent rules.

"The argument to eliminate the must-carry rules was an argument which benefited significantly the cable industry," said Charles Ferris, chairman of the FCC from 1977 to 1981. "It was a very significant issue for the cable industry."

Although cable companies offered some locally available stations to their customers, they oppose being forced to provide all locally available stations, which take up channels that could be used by stations that pay for broadcast or for which cable companies can sell their own advertising.

The antitrust division, under Ginsburg's direction, also argued in July 1986 against a proposal that the FCC consider imposing "multiple ownership restrictions" on cable operators, prohibiting companies from owning multiple systems that serve more than half the cable subscribers within a state or more than 25 percent nationwide.

"With respect to regulatory filings such as the must-carry proceeding and the Satcom {multiple ownership} proceeding, Mr. Ginsburg was as a matter of course not involved in the development or articulation of the antitrust division's position. His name was typed on the filing, and he was informed of the position taken, but he was not the signatory official," Eastland said.