HAVANA — The Cuban government on Wednesday selected 57-year-old First Vice President Miguel Mario Diaz-Canel Bermudez as the sole candidate to succeed President Raul Castro in a transition aimed at ensuring that the country’s single-party system outlasts the aging revolutionaries who created it.

The certain approval of Diaz-Canel by members of the unfailingly unanimous National Assembly will install someone from outside the Castro family in the country’s highest government office for the first time in nearly six decades.

The 86-year-old Castro will remain head of the Communist Party, designated by the constitution as “the superior guiding force of society and the state.” As a result, Castro will remain the most powerful person in Cuba for the time being. His departure from the presidency is nonetheless a symbolically charged moment for a country accustomed to 60 years of absolute rule first by revolutionary leader Fidel Castro and, for the last decade, his younger brother.

Facing biological reality but still active and apparently healthy, Raul Castro is stepping down as president in an effort to guarantee that new leaders can maintain the government’s grip on power in the face of economic stagnation, an aging population and increasing disenchantment among younger generations.

“I like sticking with the ideas of President Fidel Castro because he did a lot for the people of Cuba, but we need rejuvenation, above all in the economy,” said Melissa Mederos, a 21-year-old schoolteacher. “Diaz-Canel needs to work hard on the economy, because people need to live a little better.”

State salaries average $30 a month.