A Japan Air Lines maintenance official who had been negotiating compensation payments with families of the 520 victims of the crash Aug. 12 of a J.A.L. jet died today of knife wounds in an apparent suicide.

A police spokesman said the official, Hiroo Tominaga, 59 years old, was found bleeding from neck and chest wounds in his home in Yokohama.

A four-inch-long knife was found near his body.

A note nearby said, ''I am atoning with my death.'' The police said they believed Mr. Tominaga had committed suicide.

A J.A.L. spokesman said Mr. Tominaga had been assigned since Sept. 1 to assist the relatives of the victims. Before taking up the assignment Mr. Tominaga had coordinated maintenance on J.A.L. planes at Tokyo's Haneda airport. The Boeing 747 that crashed took off from Haneda.