A sheet-like member (14) is wound around an outer circumferential surface of a predetermined portion of a wire bundle (12) of a wire harness (1), and both side parts (141, 142) of the sheet-like member (14) are superposed on an outer circumferential surface of a predetermined portion of the wire bundle (12). A jig (5) is disposed between the outer circumferential surface of the predetermined portion of the wire bundle (12) and an inner circumferential surface of the superposed side parts (141, 142) of the sheet-like member (14). A pressurizing member (6) of an ultrasonic welder pressurizes at least part of the superposed portion of the sheet-like member (14) for ultrasonic welding, thereby forming a protector (13) from the sheet-like member (14) on the outer circumferential surface of the predetermined portion of the wire bundle (12).