When a guide wire 5 is maximally raised on a treatment instrument raising stand 35, a distal point of the guide wire 5 is operated so as to be capable of always observing a papillary opening portion 7, which is a subject to be treated, from the front (in a basic position where the portion to be treated is positioned at the center of the field of view). The treatment instrument raising stand 35 is maximally raised in order that the tilt angle β of the guide wire 5 is equal to the tilt angle α of a central axis of a field of view of an observation window 30. Then, the guide wire 5 is fixed and a treatment instrument such as a drainage tube is replaced, whereby a treatment instrument can be easily and more safely replaced via the guide wire 5.
