ANNAPOLIS, AUG. 14 -- Maryland's highest court ruled today that a Montgomery County man who paid a $45 fine after he was involved in a traffic accident in which a popular junior high school principal was killed may not now be tried for manslaughter.

The Court of Appeals, overturning the ruling of a Montgomery Circuit judge, unanimously held that the indictment of Rockville resident Frank R. Gianiny, 29, on a vehicular manslaughter charge violated his constitutional protection against double jeopardy. Both the state and federal Constitutions provide that people may not be prosecuted twice for the same offense.

The indictment was handed up in February 1989 days after Gianiny, who was aware of the pending grand jury action and acting on the advice of his attorney, paid a traffic citation for negligent driving. The citation was issued at the time of the accident on Nov. 20, 1988, that resulted in the death of Mark D. Mann, 42, principal of Parkland Junior High School.

In its opinion, the appellate court said today that under Maryland law, negligence and manslaughter are similar offenses that would require the same evidence to support a conviction. Therefore, because Gianiny's payment of the fine constituted a conviction, his trial under the indictment was barred as double jeopardy.

According to police and prosecutors, Mann was killed on Greenspan Lane in Rockville after Gianiny's Jeep went out of control and slammed broadside into the principal's stopped car. They allege that Gianiny was traveling at more than twice the speed limit of 25 miles an hour in the rain when the accident occurred. Gianiny's attorney, Michael McAuliffe, says his client was going 30 miles an hour and had been run off the road by another driver.

Mann's widow, Ellen Steinberg Mann, said she was deeply upset by what she called the loophole that enabled Gianiny to escape the more serious manslaughter charge. Conviction of that offense carries up to five years imprisonment and a $1,000 fine.

"I really felt very strongly that he should be punished," Mann said, adding that she has been working with a Montgomery County legislator to amend the manslaughter law. "He is going on with his life after paying a $45 fine and I'm left with three children who have this devastating loss. I don't think any portion of society wants that."

Assistant Attorney General Richard B. Rosenblatt said he was disappointed, but not surprised by the ruling. "Obviously, we could have avoided everything if a ticket was not issued," he said.

Sgt. Harry Geehreng, a Montgomery County police spokesman, said that after the Mann case and a similar incident last year, the county state's attorney has informally asked officers not to issue citations in fatal accidents without consulting prosecutors.