Mr. Ryan, 73, told reporters that he faced prison with a clear conscience.

“I have said since the beginning of this 10-year ordeal that I am innocent,” he said. “And I intend to prove that.”

Mr. Ryan, who in 40 years in public office became one of the most powerful Republicans in the Midwest, was convicted last year of a long list of corruption charges stemming from his tenure as secretary of state and governor of Illinois, including using public money for campaign work and exchanging state business for money and gifts, among them an island vacation.

Outside Illinois, he was better known for his moratorium on the death penalty and commuting more than 160 death sentences to life in prison just before leaving office after one term, in 2003. To some, Mr. Ryan’s prison term should be cautionary in a state where making deals and giving favors have long been viewed as politics as usual. He is the third former Illinois governor convicted of wrongdoing.

“I would like to believe that this will have a chilling effect on corruption by public officials, but I’m not optimistic,” said Mike Lawrence, who directs the Paul Simon Public Policy Institute at Southern Illinois University.

New accusations of corruption continue to swirl around officials in the state. Federal investigations have started in connection with officials in Springfield, the capital, as well as Chicago City Hall.

Former Gov. James R. Thompson, a member of Mr. Ryan’s legal team, said he would appeal the case to the Supreme Court next year. A federal appeals court upheld the conviction in August.

Before dawn on Wednesday, Mr. Ryan left his home in Kankakee, 50 miles south of here, his face lighted by the news camera flashes. They later filmed him entering a pancake house in Chicago. Arriving at the prison in Oxford, Wis., Mr. Ryan entered through a back door.

The prison has housed other Illinois politicians, including Chicago aldermen, a Cook County Circuit Court clerk and former Representative Dan Rostenkowski.

Mr. Thompson suggested that Mr. Ryan would work a prison job, as required of all healthy inmates. Prison officials say possible jobs include serving food, mopping floors, landscaping and cleaning toilets. Prison officials said he was allowed to take in his wedding ring and an extra pair of eyeglasses.