The guy going after Rivers is Billy Sammeth -- Joan's longtime "personal manager" -- who claims he "took [Joan's] dormant career" ... and found a way to get her more work and more money ... only to be stiffed out of $179k and humiliated in a recent documentary about Joan's life.
In the docs, Sammeth claims he never got his 10% cut from several paydays Joan received -- including a $200,000 check Joan received from winning "Celebrity Apprentice."
Sammeth also claims Joan and the people behind the documentary "Joan Rivers: A Piece of Work" -- defamed him by making him appear to be a "disappeared" manager.
Sammeth is suing both Joan and the production company for at least $2.2 million.
UPDATE: A rep for Joan tells us the comedian is, "confident that the judicial system will dispose of Mr. Sammeth's lawsuit in the appropriate manner."