Le Pen told a French TV station he did not regret making the statement repeatedly over nearly three decades, earning convictions for hate speech along the way, and then said it again.
"I have said what I believed. Gas chambers were a detail of the war, unless we accept that the war is a detail of the gas chambers," the European Parliament member told BFMTV.
"I have persisted because I believe that it's the truth and that no one should be shocked," the National Front (FN) founder added.
He first called the gas chambers a detail of World War II in 1987 and repeated it 10 years later in Munich, Germany, again in comments to a French magazine in 2008 and a year later at the European Parliament.
A national Jewish union and anti-racism campaigners condemned the statement, with activist group SOS Racisme pledging to see that Le Pen was again convicted" for his words.
Even the far-right leader's daughter and current FN president Marine Le Pen spoke out against her father's words.
"This is nothing new. I profoundly disagree with Jean-Marie Le Pen both in form and content," she told AFP.
Since taking over the FN in 2011 she has sought to make the party a more palatable alternative to mainstream voters.
It helped lead the FN to first place in last years European elections in France and a quarter of the votes in the first round of local polls two weeks ago.
The anti-immigration and anti-European Union party is steadily building a national political support base ahead of Marine Le Pen's presumed run for president in 2017.
A rift between Le Pen and his daughter widened last June over an apparently anti-Semitic pun he made in a video.
Jean-Marie Le Pen stirred up controversy in a clip posted on the FN's website in which he vowed to put his critics in their place -- including French singer Patrick Bruel, who is Jewish -- using a pun suggesting Nazi gas chambers.
Marine Le Pen immediately denounced the statement, the first time she criticised her father in public.