"The provisional detention is a necessary measure to ensure the security of the witnesses [and] the charged persons, to preserve evidence, to ensure the presence of the charged person during the proceedings, and to preserve public order," Judge Prak Kimsan said.
"There is a risk that he will disappear," he added.
Duch's defence lawyers had argued last week that he should be freed because his human rights had been violated by the more than eight years he has already been in jail without trial.
The former boss of the notorious S-21 torture and interrogation centre is awaiting trial on crimes against humanity, expected to begin next year
During last week's hearing prosecutors said that Duch's release would pose a threat to public order in Cambodia, and he could attempt to escape justice by fleeing the country.
They argued that he should remain behind bars for his own safety because he could be harmed both by "accomplices wishing to silence him and by the relatives of victims seeking revenge".
Duch showed no emotion as the decision was read out, although a member of his legal team later said they were "frustrated" at the ruling.
His case was the first heard before a UN-backed tribunal for former Khmer Rouge members and had been seen as a key test of the court's credibility.
Duch was initially arrested by the Cambodian government in 1999. Last year he was handed over to the tribunal and charged with crimes against humanity.
He is one of five former senior Khmer Rouge leaders currently in detention awaiting trial before the tribunal.
During the Khmer Rouge's rule over Cambodia between 1975 and 1979 Duch was in charge of the Khmer Rouge's secret police as well as the notorious S-21 torture and interrogation centre at Phnom Penh's former Tuol Sleng high school.
|Duch appeal was the first case to be heard |
by the UN-backed tribunal [AFP]
Some 17,000 people are thought to have passed through the centre, with only a handful surviving.
Among the five former officials awaiting trial Duch is the lowest ranking member of the former regime, but his role in charge of S-21 has made him one of the most notorious.
He has insisted he was simply following orders from the top to save his own life.
"I was under other people's command, and I would have died if I disobeyed it," he told a government interrogator after his arrest.
The first formal trials before the tribunal are expected to get under way early next year.