WASHINGTON -- Norman Y. Mineta, the only Democrat in President Bush's Cabinet, has resigned as transportation secretary, the White House said yesterday.

In a letter to Mr. Bush, Mr. Mineta, 74, said that after "five productive and memorable years, it is time for me to move on to other challenges."

Mr. Mineta became the first Asian American to serve in a White House Cabinet when President Bill Clinton named him secretary of commerce in Mr. Clinton's final year in office. Mr. Mineta stayed on in the Bush administration to work on transportation, an interest of his during a 30-year career in Congress.

His departure, effective July 7, leaves only two secretaries from the original Bush Cabinet still in place: Defense Secretary Donald H. Rumsfeld and Labor Secretary Elaine Chao. In a capital riven by partisan divisiveness, Mr. Mineta was fond of saying, "There are no Democratic or Republican highways, no such thing as Republican or Democratic traffic congestion."

Mr. Mineta was at the Transportation Department helm on Sept. 11, 2001, overseeing the chaos that resulted when all civilian planes were grounded for the first time in the nation's history. In a written statement yesterday, Mr. Bush said Mr. Mineta had performed "a crucial role" that day, "leading the successful effort to bring tens of thousands of passengers aboard commercial aircraft to safe landings.

"His leadership was vital to strengthening the security of America's seaports and airports. ... After Hurricane Katrina, Norm and his team were able to rapidly repair and reopen the region's major highways, airports, seaports and pipelines," Mr. Bush said, pointing out that Mr. Mineta is the longest-serving transportation secretary in U.S. history.

In announcing the resignation yesterday, White House Press Secretary Tony Snow said Mr. Mineta "was not pushed out. As a matter of fact, the president and the vice president and others were happy with him. He put in 5 ¹/₂ years -- that's enough time."

In his resignation letter, dated June 20, Mr. Mineta said he was proudest that his department, within a year of the Sept. 11 attacks, had developed the new Transportation Security Administration to protect air passengers. He praised a little-known aspect of the Transportation Department's work: helping to rebuild the transportation infrastructure in Afghanistan and Iraq.

A native of San Jose, Calif., Mr. Mineta was 10 when he and his family were imprisoned in a Japanese internment camp during World War II. They were among the 120,000 Japanese Americans held in camps while the United States was at war with Japan. Wearing his Cub Scout uniform and carrying a baseball mitt and bat (which officials confiscated), Mr. Mineta was taken to a holding facility before being transported to a camp in Wyoming.

After the war, the Japanese-American community in California decided to protect against future persecution by grooming politicians. Mr. Mineta was one of their first success stories.

After graduating from the University of California-Berkeley, Mr. Mineta joined the Army in 1953, serving as an intelligence officer in Japan and Korea. He served on the San Jose City Council and, in 1971, became the city's mayor, the first Asian American to run a major U.S. city.

As mayor, Mr. Mineta honed the transportation philosophy that would guide his career, advocating control of transportation decisions by local government.

When he was elected to Congress in 1975, Mr. Mineta won a spot on the House Public Works and Transportation Committee, becoming chairman in 1992. He was an author of the landmark Intermodal Surface Transportation Efficiency Act of 1991, which shifted decisions on highway and mass-transit planning to state and local governments. The legislation led to upsurges in mass-transit ridership and greener transportation options, like bike paths.