A Washington lawyer, appointed by a court to represent the estate of an elderly D.C. man who was no longer able to conduct his own affairs, pleaded guilty yesterday to defrauding the man of more than $376,000.

Francis P. Noonan, 63, who has offices here at 1999 Vermont Ave. NW, entered the plea before U.S. Senior District Judge George L. Hart Jr.

Assistant U.S. Attorney Eric B. Marcy said Noonan used the stolen funds to pay off debts he had incurred on a Poolesville, Md. horse farm he owns. Noonan will be sentenced later.

Noonan was appointed by D.C. Superior Court in 1976 to represent Joseph M. Costa, who the court found to be incompetent to handle his own affairs.

Costa, the former owner of a liquor store here, lives with his brother in a large house on upper 14th Street NW. According to a witness who has been inside the house, the two men live a virtual recluses and the house is furnished much like it must have been furnished decades ago.

According to court documents, Costa is extremely wealthy. It takes more than 50 legal-size pages to merely list the total number of dividend checks he receives from a wide range of stocks he owns.

Many of the dividend checks have gone uncashed and are stored in boxes, one investigator said.

According to the charges placed against Noonan, the lawyer was supposed to maintain trust accounts for Costa and supply him with living expenses. Noonan would carry food to the house for the Costa brothers, according to one investigator.

Instead of properly maintaining the accounts, according to court papers, Noonan would cash checks intended for Costa and put them into his own accounts at several Washington banks. He then would use the money for his own purposes taking a total of $303,400 from Costa's saving accounts, the papers said.

Noonan hid his scheme from the D.C. Superior Court, to which he had to make regular reports, by failing to accurately document his activities, the papers continued.

According to a court audit, Noonan made the withdrawals from Costa's accounts on an almost weekly basis for about two years with the amounts of the withdrawals ranging from $750 to $15,000 at a time.

Complete restitution has been made to the Costa estate by a bonding company, which in turn has taken a deed-of-trust on Noonan's Poolesville horse farm, investigators said.

A new administrator has been appointed for the Costa estate, but the Costa brothers reportedly are still friendly with Noonan and he reportedly still takes them food regularly.