As a builder, Kongo Gumi is the exception rather than the rule for millenia-old companies. When you look at the list Robin points to above, the descriptions read "Hotel, Restaurant, Wine, Brewery, Brewery, Brewery..." The vast majority are either makers or servers of alcohol (or both).

But I love the two European paper companies from the Middle Ages: Sweden's Stora Enso and France's Richard de Bas. That's pre-Gutenberg, son! Odds are pretty good that these businesses shifted production from parchment to linen paper to wood pulp as the centuries rolled on, and as paper went from a luxury good to perhaps the most ubiquitous part of our modern reality.

According to Wikipedia, the University of Al Karouine in Morocco is the oldest continually operating educational institution in the world. It was founded in 859.

Al-Azhar in Cairo is next, followed by a string of European universities: Bologna, Paris, Oxford. I can't help but think that there might be an ancient Chinese, Japanese, or Indian school that isn't represented here, given the Western slant of this list.

Well the thing w/ Chinese and Japanese schools is that I feel they would almost certainly have been shut down in one of those countries' huge schismatic revolutions. I mean... I guess Europe had its share of schismatic revolutions, too. But nothing quite like Mao's China in terms of a determined cultural reboot, right?

Al-Azhar is a really interesting place -- it got a lot of attention in Reza Aslan's book "No god but God" -- I'd love to read or see something more detailed on its history and/or current status.