noun
Formal or legal term for the person who owns something. Often interchangeable with 所有者, but 所有主 can emphasize the person as the master or holder.
See also: 所有者
この土地の所有主は誰ですか。
Who is the owner of this land?
所有主の許可なく立ち入ることは
You cannot enter without the owner's permission.