Margen
In the year 578, Prince Shōtoku brought a carpenter named Shigemitsu Kongō over from the Korean kingdom of Baekje to build the Shitennō-ji temple in Osaka. When the work was done, the carpenter stayed, founded a company, and his family kept building and repairing temples for some forty generations.
Kongō Gumi survived everything: centuries of civil war, fires that burned down its own temples (they rebuilt them), the persecution of Buddhism in the nineteenth century, and two world wars — during the second, they got by making coffins. Their secret wasn't size but patience: choosing the most capable heir (not always the eldest son; sometimes an adopted son-in-law) and almost never straying from what they knew how to do.
They broke their own rule exactly once. In the 1980s, at the height of Japan's bubble, they borrowed heavily to invest in real estate. When the bubble burst, the debt slowly ate them, and in 2006 the world's oldest company was absorbed by a construction firm... founded in 1917.
The name still exists, and it still does the same thing: temples. Fourteen centuries of craft fit into one lesson — what kills you is almost never your business; it's other people's.