LOS ANGELES (AP) _ A security guard pleaded innocent to arson in the Universal Studios backlot fire, and his lawyer said the man is ''very, very distraught'' and may have a mental disability.

Michael John Huston, 40, acknowledged his name but did not speak during a brief arraignment Thursday. Municipal Judge Candace Beeson set his next court appearance for Dec. 13 and ordered him held on $300,000 bail.

Huston's grandmother and mother both have told reporters that he admitted to them he had started the fire. The grandmother, Dorothy Montanari, quoted Huston as saying, ''You saw the fire on TV; I did it and I'm going to jail.''

Outside court, his attorney, Charles English, said he was investigating the reports about a mental problem stemming from service in the Vietnam War.

''There are a lot of different levels of mental disability,'' he said. ''We'll have to look at all of Michael's states and see what's going on.'' He described Huston as ''very, very distraught.''

''He's anxious to get this moving and find out the evidence against him,'' English said.

Deputy District Attorney Jane Blissert said witnesses have placed Huston at the fire scene shortly after it started. She responded to questions about his sanity by saying, ''He has the mental capacity to commit these crimes.''

She said investigators determined the fire was sparked by an open flame such as a match or cigarette lighter.

The Nov. 6 blaze wiped out three acres of film history, destroying make- believe streets and facades which served as backdrops for movies including ''Dick Tracy,'' ''Back to the Future'' and ''Dirty Harry.'' It also damaged parts of the Universal Studios Tour, the third most popular tourist attraction in the nation.

Blissert said she didn't know what motive Huston might have had for setting the fire.

Huston was an employee of Burns International Security Services, a New Jersey company that has a contract with Universal. He had been on the job one month when the fire occurred, the studio said.