What is "Marugen" ?

[Maciamo]

Does anybody know what the "Marugen" signs on so many buildings in Tokyo mean ? The signs are always the same, with the kanji 源 (gen) inside a circle (maru), then athe name in romaji and a number. There are a few Marugen between Ginza and Shimbashi, in Akasaka and other places in Tokyo.

I've taken this picture in Akasaka. Marugen could be the owner or builder name. I don't think it's a company. I've asked my wife and other people but nobody knows.