PITTSBURGH (AP) _ An imprisoned lawyer sued cable television network ESPN on Friday, claiming he was defamed during the Nov. 23, 1987, broadcast of a game between the San Francisco 49ers and the Cleveland Browns.

Alan Frank, 52, of Pittsburgh was convicted of federal tax evasion during the summer. He claims his reputation was damaged during the broadcast. His son, John, plays for the 49ers.

The suit filed in Allegheny County Common Pleas Court says that during the first quarter of the nationally televised game, an announcer made defamatory remarks about the elder Frank in connection with his son's finances.

Eric Kemmler, associate general counsel for ESPN, said Frank wrote a letter about the broadcast soon after it appeared and complained he had been libeled.

''We reviewed the situation and found that information our reporter used was reliable,'' Kemmler said. ''We conferred with the prosecutors involved in the cases. It's preposterous that he's filing a libel suit.''

Frank is serving six years for individual tax evasion and conspiring to avoid income taxes on money derived from Acme Music Co., a Pittsburgh vending machine business.