The director of a Northern Virginia mental health center that President Carter and an entourage of VIPs visited recently for a bill-signing ceremony has resigned in the middle of an investigation by Fairfax County prosecutors into possible violations of state conflict-of-interest laws.

Dr. Allan Conway, the 35-year-old director of the Woodburn Center for Community Mental Health in Annandale, one of the largest and most prestigious mental health facilities in the metropolitan area, has announced he is leaving his $50,000-a-year post next month for an undisclosed job in California.

Conway said yesterday that his departure from the position he has held for three years is unrelated to a probe into the center's contracting and financial procedures begun last July by Fairfax County prosecutor Robert F. Horan Jr. Horan said that thus far he has found no evidence of any criminal wrongdoing. The center, which has an annual budget of $3 million and sees 3,000 patients a month, also is the subject of an audit ordered by other county officials.

Horan, a Democrat, said that on the day before the president's Oct. 7 visit to Woodburn -- billed by the White House as a nonpolitical trip -- he received a call from a member of the White House staff about the probe. "They asked me is anybody going to be indicted and I said no, that there were these allegations but there was no cloud," said Horan, who in 1972 unsuccessfully ran for Congress from the 8th Congressional District. "Evidently somebody had the good political sense to make an inquiry. I guess they wanted to make sure nobody was going to be arrested when the president walked in."

As he signed the bill designed to improve delivery of mental health care, the president said he was "particularly glad to come here to a community mental health center where people have been served for 30 years."

White House scheduling officials contacted yesterday said they knew nothing of the probe or the phone call to Horan. "I don't see why it would be relevant anyway," said Conway, who was present at the bill-signing ceremony attended by Rosalynn Carter, Sen. Edward M. Kennedy (D-Mass.) and Health and Human Services Secretary Patricia Roberts Harris.

Horan said his investigation into more than 100 personal service contracts and what he called the "unusual" and complex financial relationship between Woodburn and two private non-profit corporations set up by the center has revealed no improprieties. The prosecutor said he probably will conclude his probe after the conclusion of the audit, scheduled for release in the next several weeks.

"The indications are that these arrangements were a rather clever device to take advantage of certain federal grants," said Horan, whose inquiry was prompted by an anonymous letter sent to county officials and the press. "The bona fide intent was to provide more mental health services."