The court also ordered Abdul Wadud Bhuiyan's property confiscated, public prosecutor Mosharraf Hossain Kazal said.
"The judge in his verdict said there is a gross mismatch between the income and wealth of Bhuiyan.
He was unable to provide clear evidence how he acquired the fortune," he said.
Bhuiyan, 46, an influential former Bangladesh Nationalist Party (BNP) member of parliament, was arrested in April by a military-backed emergency government.
Bangladesh has been under emergency rule since January when a political crisis over poll rigging allegations led to elections being cancelled.
The new government has since launched a massive corruption crackdown aimed at cleaning up the country's notoriously tarnished politics before holding fresh elections by late 2008.
It has arrested more than 150 prominent politicians, businessmen and bureaucrats from both the BNP, which led a coalition government until last October, and the former main opposition Awami League.
Last week former