Dr. Frank B. Ryan, a National Football League quarterback turned computer expert, quietly resigned yesterday as director of the House Information Systems.

Ryan, 40, who held the job since Arpil, 1971, announced his resignation in a letter to Rep. Frank J. Thompson Jr. (D-N.J.), chairman of the House Administration Committee. The committee has jurisdiction over the computer operation.

Ryan said in an interview that his "main reason" for resigning was to "move on to other things." He said he felt he had "sort of reached a plateau" in the job. He is said to have played a major role in introducing computer technology to Capitol Hill.

However, the resignation was marked by rumors of political maneuvering on Thompson's part. It came in the wake of two House-commissioned studies that were critical of the operation.

Ryan's job has temporarily been taken over by Boyd Alexander, former director of computer services at the Office of Management and Budget.

Thompson deferred comments on the matter to Rep. Charles Rose (D-N.C.), chairman of the committee's Policy Group on Information and Computers, which sets policy for HIS.

Rose dismissed allgations that politics were involved in Ryan's departure.

"Chairman Thompson wanted Frank Ryan and his staff to attemp to correct some of those problems that were pined out' in the House-commissioned studies, Rose said.