The Justice Department is investigating Defense Department ex-official Michael E. Pillsbury, who was fired for allegedly leaking classified information to the press, knowledgeable officials said yesterday.

The decision to probe further into the Pillsbury case is the latest development in an apparent crackdown by the Reagan administration on the steady flow of unauthorized information from the government.

Pillsbury, a political appointee, was dismissed Monday from his job as assistant undersecretary of defense for policy planning after he did not pass a polygraph test, according to Pentagon sources. The sources said the test was part of an effort to find the source of a leak to The Washington Post last month about a Reagan administration decision to supply hand-held Stinger ground-to-air missiles to anticommunist rebels in Angola and Afghanistan.

Although officials yesterday declined to comment further about the Justice Department investigation, some pointedly recalled the case of Samuel Loring Morison, a former Navy intelligence analyst, who was sentenced in December to two years in prison for sending classified satellite photographs to the British magazine, Jane's Defence Weekly. He was the first person criminally convicted of leaking classified information to the media.

Pillsbury was unavailable and the Justice Department declined to comment.

Morton H. Halperin, director of the American Civil Liberties Union's Washington office, said the ACLU regards the use of criminal penalties for unauthorized leaks as unconstitutional and inappropriate. "A probe [by the Department of Justice] would be consistent with their interpretation of the law in the Morison case, but we think the relevant statutes relate to espionage and not to release of information to the press," he said.

Although the government has the right to fire employes for unauthorized disclosure of information, Halperin said, "we object to the use of polygraphs" because they "are not a reliable indicator."

Responding to a series of espionage cases, the Reagan administration last November authorized polygraph tests for all individuals with access to highly classified information, but narrowed the scope of the order after Secretary of State George P. Shultz said he would resign if asked to take the test.

President Reagan has complained about the relatively free flow of data from his officials, most recently in a speech to the American Society of Newspaper Editors earlier this month when he said the White House "is the leakiest place I've ever been in."

The administration and Congress also have exchanged charges about which branch of government is most responsible for unauthorized disclosures.

The Defense Department in 1981 tried to fire John Tillson, a civilian executive in its manpower and logistics office, after he failed three polygraph tests regarding the leak to The Washington Post of a Pentagon briefing on the possibility that Reagan's rearmament program could produce a $750 billion cost overrun over five years.

Tillson fought successfully to keep his job and later was recommended for an outstanding service award. Washington Post reporter George C. Wilson later said publicly that Tillson was not the source of his report.

Leaks are common sources of news articles from every branch of government.