An adapter for an endoscope of the present invention includes: a locking device for attaching and detaching that attaches and locks this adapter for an endoscope onto a predefined position on an endoscope control part; and a first treatment tool locking device that keeps a treatment tool at a first predefined position. Furthermore, at the first predefined position, a guidewire entry aperture of the treatment tool and an entry aperture of a forceps plug are separated in such a way as to face each other along substantially the same line.