In a fuel supply system, a fuel pump supplies fuel from a fuel tank to a fuel supply piping and to a back pressure introducing passage. A pressure regulator has a fuel pressure regulating chamber communicated to the fuel supply piping, and a back pressure chamber communicated to the back pressure introducing passage. When a fuel pressure in the fuel pressure regulating chamber is larger than a relief pressure that bulges the diaphragm toward the back pressure chamber, a relief valve opens a relief port to return the fuel from the fuel pressure regulating chamber to the fuel tank. The relief pressure is adjusted by controlling a fuel pressure in the back pressure chamber of the pressure regulator.