Tin Oo, 80, was ordered confined to his Yangon home for another year, a Myanmar official told AFP on condition of anonymity.
Tin Oo is the deputy leader of Aung San Suu Kyi's National League for Democracy (NLD) party. The international community, as well as local activists, have repeatedly called for the pair to be released.
He was arrested along with the Nobel peace prize winner in May 2003, after a pro-government mob attacked their convoy during a political tour of the country's north.
The two led the NLD to victory in 1990 elections, but the junta never recognised the result.
Tin Oo was initially held in prison in Kalay in the remote northwest, but in February 2004 was brought back to his home in Yangon, where he has been held ever since.
More than 1,800 political prisoners are believed detained in Myanmar, including about 700 held over pro-democracy protests led by Buddhist monks in September.
Security forces violently suppressed the protests, leaving at least 31 dead according to the United Nations.
Myanmar's military, which has ruled since 1962, has come under mounting international pressure to open a dialogue with Aung San Suu Kyi to move towards national reconciliation.
But the regime has insisted on sticking to its self-proclaimed "road map" to democracy, and on Saturday unexpectedly announced that it would hold a referendum in May ahead of democratic elections in 2010.
Critics warn any balloting would be meaningless with Aung San Suu Kyi, Tin Oo and other democracy leaders locked away. (AFP)
Myanmar extends arrest of Aung San Suu Kyi's deputy: official