Mineta, who oversaw the huge transportation security buildup after the Sept. 11, 2001, terrorist attacks, had been plagued at times by back problems and spent months working from home and the hospital. But he has since recovered.
He is "moving on to pursue other challenges," his spokesman, Robert Johnson, said Friday.
White House press secretary Tony Snow announced the resignation. Asked why Mineta, 74, decided to leave, Snow said: "Because he wanted to."
"He was not being pushed out," Snow said. "As a matter of fact, the president and the vice president and others were happy with him. He put in five and half years -- that's enough time."
Snow credited Mineta with establishing the Transportation Security Administration, cutting regulations and red tape to liberalize the commercial aviation market, helping shape the legislation that finances the nation's highways, and injecting "sound economic principles" into the nation's passenger rail system.
Snow also paid tribute to Mineta's long history in public life: his service in the Army, his elections to local positions in California, his 20 years representing California in the U.S. House, and his tours in two Cabinet positions, the first as commerce secretary under former President Clinton.
Mineta joined Bush's Cabinet on Jan. 25, 2001, and became Transportation's longest-serving secretary. Bush's only other two original Cabinet members still serving are Defense Secretary Donald H. Rumsfeld and Labor Secretary Elaine Chao.
There had been speculation for years that Mineta was on the verge of resigning, sometimes because of his health and sometimes because of Cabinet shake-ups.
The Transportation Department changed dramatically during his tenure, swelling to 160,000 people when the Transportation Security Administration was created in 2002, then shrinking to fewer than 60,000 when the TSA and the Coast Guard left to become part of the newly created Homeland Security Department.
One of Mineta's main achievements was the passage of a six-year, $286.4 billion highway spending plan in July, after nearly two years of wrangling. The plan has since been criticized for containing too many "earmarks," special projects sought by lawmakers.
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A number of firsts
The son of Japanese immigrants, Mineta's career has been a series of firsts for Asian-Americans: first to serve as a Cabinet secretary when Clinton appointed him in 2000; first to serve as mayor of a major city -- his native San Jose, Calif., where the airport bears his name; and first to chair a congressional committee, the House Transportation Committee.
After terrorists hijacked airplanes for the Sept. 11 attacks , Mineta oversaw the creation of the TSA, which put thousands of air marshals on commercial flights, installed high-tech equipment to check baggage at airports and hired tens of thousands of workers to screen air travelers and their baggage.
One of Mineta's signature accomplishments in the House was passage of the Civil Liberties Act of 1988, through which the United States apologized for sending Mineta and 120,000 other Japanese-Americans to internment camps and paid reparations of $20,000 to each survivor.
Mineta was 10 when he was herded to a camp in Wyoming with his family after Japan attacked Pearl Harbor in December 1941.
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