SAN FRANCISCO (AP) _ A federal appeals court on Thursday overturned the wartime conviction of a Japanese-American for resisting curfew and violating a military internment order, saying the government had suppressed crucial evidence.

The ruling by the 9th U.S. Circuit Court of Appeals in the case of Gordon Hirabayashi follows similar decisions by lower federal courts in his case and another case, which both relied on the same disclosure - evidence that the imprisonment of 120,000 Japanese-Americans during World War II was done for racial rather than military reasons.

It was the first time a federal appeals court had ruled on the question.

Hirabayashi, a 23-year-old University of Washington senior at the time of his arrest in 1942, and now a 69-year-old retired sociology professor in Edmonton, Alberta, said the ruling was ''very good news.'' However, he said he wanted the decision to be upheld by the Supreme Court.

Noting that Reagan administration officials had offered to drop the convictions but opposed his effort to get a court ruling, Hirabayashi said, ''It would be disappointing to me if they suddenly lost their incentive'' and did not appeal.

Justice Department spokesman John Russell declined comment on the ruling and said a decision on an appeal to the Supreme Court was weeks away.

Among other things, the department had argued that the case was moot because the convictions cause no current harm to Hirabayashi. Judge Mary Schroeder disagreed, saying, ''A United States citizen who is convicted of a crime on account of race is lastingly aggrieved.''

Hirabashi spent a year in custody following his arrest and later convictions for violating a military order to report to a control center as the first step toward placement in an internment camp, and for violating curfew. The case is an ''attempt to prevent this sort of thing from happening again,'' Hirabayashi said.

''The Constitution has guaranteed my protection regardless of race, sex, religion and so on, but it doesn't guarantee that it's going to be applied. The citizens have to be vigilant to see that the Constitution is going to be applied in all cases.''

The key document in the case, unearthed in 1982, was the original version of a 1942 report by the military commander on the West Coast, Lt. Gen. John DeWitt.

In it, DeWitt used a racial stereotype to buttress his argument that all people of Japanese ancestry would have to be evacuated until the end of the war because it would be impossible to separate the loyal from the disloyal.

The report did not say the action was needed because the military emergency allof the Supreme Court would probably have been profoundly and materially affected if the Justice Department had advised it of the suppression of evidence,'' Schroeder said in the opinion for a unanimous three-member appeals court panel.

Out of more than 110,000 Japanese-Americans held in camps during the war, only a handful - perhaps fewer than 50 - were charged with violating curfew or other military orders, said Donald Tamaki, one of a team of lawyers that has worked to overturn the cases. He said Hirabayashi was one of only three to make constitutional challenges, which were rejected by the Supreme Court in 1943 and 1944.

''This was a besieged minority,'' Tamaki said. ''After Pearl Harbor an atmosphere of terror descended on the Japanese community.'' He said community leaders were promptly arrested by the FBI, and many residents feared being deported or even shot.

The court upheld a ruling by U.S. District Judge Donald Voorhees of Seattle overturning Hirabayashi's conviction for violating the order to report to a control center. Voorhees had let stand Hirabayashi's conviction on the lesser charge of violating a curfew imposed on Japanese-Americ ans, but the appeals court disagreed and ordered that conviction reversed also.

The only other case to be decided by a federal court was not taken by the government to the appeals court level. In 1984, U.S. District Judge Marilyn Hall Patel of San Francisco overturned the conviction of Fred Korematsu.

Korematsu's case was the basis of the 1944 Supreme Court ruling upholding the internments; the previous year, the court had upheld the curfew orders in Hirabayashi's case and a companion case.