A Fairfax County lawyer, who is eight days away from becoming a full-time judge, conceded yesterday that he mistakenly dismissed a criminal charge against a 17-year-old youth while sitting as a substitute judge.

Frank B. Perry III, in a letter released yesterday, ordered the reckless driving charge reinstated. He had dismissed the charge last year by signing the wrong court papers, prosecutors said.

Perry's decision is expected to be challenged by the youth's lawyer, who contended in a court hearing this week that Virginia judges must correct their mistakes within 30 days of their occurences.

"This is just an inadvertent error, not an error of judgment," said Commonwealth's Attorney Robert F. Horan Jr. He asked Perry to reopen the case after receiving complaints from the family of a young man who was killed in an accident with the youth.

Horan had argued that Perry had "no recourse except to rescind the dismissal" because it was inadvertent.

The error was not discovered until three months after Perry signed an order dropping the charge growing out of the fatal accident. Horan said the judge had intended to drop a separate reckless driving charge against the same youth, but mistakenly signed papers in the fatal accident case.