A Belgian citizen pleaded guilty yesterday to commercial espionage, admitting in federal court that he attempted to pay $500,000 to obtain secret computer technology for the Soviet Union.

Under a plea bargain with federal prosecutors in Alexandria, Marc Andre DeGeyter acknowledged arranging for the illegal sale of tightly guarded data that would act as a decoding device for a widely used computer program made by the Software AG company in Reston.

DeGeyter also pleaded guilty to paying $500,000 to an FBI undercover agent posing as an employe of the Virginia firm. In return, federal prosecutors dropped six charges remaining against DeGeyter.

DeGeyter, who is free on $100,000 personal bond, faces a maximum sentence of one year in prison and $25,000 in fines on the two counts to which he pleaded guilty. In addition, he consented to pay $10,000 fine levied by the Department of Commerce in the case. Sentencing was set for Aug. 1.

According to federal prosecutors, DeGeyter, 31, of Schilde, Belgium, tried to obtain a decoding device for a data management program produced by Software. Industry sources said yesterday the system is used by such concerns as Boeing Aircraft, the Defense Department and numerous banks.

U.S. prosecutor Theodore Greenberg estimated the value of the tightly guarded decoding device at $20 million to $30 million.

DeGeyter was arrested on May 18 at New York's Kennedy International Airport after a year-long FBI investigation. During that time, DeGeyter, who claimed to be the president of a Brussels company called TVS Broadcast Systems Division, traveled to several U.S. cities and carried a passport indicating he had been to Moscow on numerous occassions, the FBI said.

An FBI spokesman said DeGeyter had approached several U.S. companies about purchasing Software AG data management materials. The FBI would not say whether any of the companies involved were customers of Software AG.

According to law enforcement officials, DeGeyter said he was working on behalf of a Soviet firm called Telemashimport, a state agency that arranges the import of high technology processes and is involved in chemical manufacturing.

Officials at Software AG were unavailable yesterday to comment on the case.

Industry specialists said DeGeyter would have been able to buy a Software AG computer program relatively easily from any number of companies. What he was trying to obtain, however, was the coding material on which the program is based. The "data source" codes are a trade secret and their sale is not permitted to the Soviet Union, they said.

Had DeEyter been successful in obtaining the "data source" decoding tapes, computer specialists said that he, or whoever was using them, would be able to learn how the Software AG program was being used elsewhere, or how it might be used.

The FBI first learned of DeGeyter's efforts to buy the "data source" instructions in May 1979, an FBI spokesman said. The agency tracked DeGeyter's travels to Birmingham, Ala., Laguna Beach, Calif., New York and cities in Missouri and Pennsylvania. At least one company official whom DeGeyter approached participated in the investigation, prosecutor Greenberg said.

On April 16 of this year, DeGeyter was introduced by an employe of one of the companies he had approached to an FBI agent posing as an employe of Software AG. Over dinner at the Holiday Inn in Rosslyn, they discussed the possible purchase of the "data source" material, the FBI said. d

The FBI agent told DeGeyter it was not for sale, that he would have to "steal it," the spokesman said. Two days later DeGeyter arrived from New York on the Eastern shuttle and met the agent at National Airport. There, the FBI spokesman said, DeGeyter offered the agent $250,000 for the "data source" tapes.

One mouth later, the same agent met DeGeyter again at Kennedy airport in New York. The FBI spokesman said DeGeyter handed the agent a check from a Brussels bank in the amount of $500,000 exchange for boxes purportedly containing computer tapes.

DeGeyter was arrested immediately. FBI officials in New York said subsequent examination of the briefcase he was carrying revealed a Soviet visa valid through May 25 and $1,900 in American Express traveler's checks.