The administrator of the General Services Administration acknowledged yesterday the existence of "serious weaknesses" in the way the agency purchases office space counsel to investigate wrongdoing within the agency.

Jay Solomon, who became GSA administrator a year ago, made the announcement at a press conference called to discuss internal and Justice Department investigations into fraudulent contracts to maintain federal buidlings and to purchase office supplies.

The investigations are into contracts to perform much more maintenance work than actually is done in federal buildings and into contracts to purchase supplies that are not delivered.

Sources familiar with the investigations say some of the building managers who approved the contracts received cash and favors from contractors, while some managers of supply centers received stereo equipment and other gifts in return for ordering supplies that never were delivered.

Soloman declined to discuss the scope of the fraud being uncovered. He said GSA could take no action against employes under investigation without jeopardizing their rights if thry are prosecuted.

He said any employe shown by GSA to have violated the public trust will face disciplinary measures by GSA.

Referring to those under investigation by Justice, Soloman said, "The movement we can be certain of the guilt of individuals, we will take appropriate action against them, including reassignments, downgrading, or outright dismissal. We aslo will urge the Department of Justice to seek restitution form any who have profited by abuse of their positions."

Those who "allowed crimes to be committed while they looked the other way" will be punished through administration action, Soloman said.

Saying GSA had referred possible criminal cases over the years to the Justice Department, Soloman said, "In view of the frustrating experience of prior administrations with these problems, it is increasingly clear to me that unless we improve the structure and systems of this agency, we can look forward to a dreary repitition of the fraudulent activities which have plagued GSA for many years."

Toward this end, Soloman said, he has appointed Vincent R. Alto, a former trial attorney in the Justice Department's organized crime section, to act as an inspector general within the agency until Congress passes legislation that would provide such an officer within each agency of government.

Alto agreed to serve in that capacity for four or five months, since the legislation is expected to become effective in September, Soloman said.

Alto will be charged with investigating "aggressively every allegation of wrongdoing, from whatever source." Soloman said. He will be in charge of GSA's internal auditors and investigators, he said.