An insertion instrument (2) which is connected to a manipulating head grip (1) is composed of an elongated flexible tube section (2a), an articular section (2b) and a rigid tip end section (2c) in series from a proximal end which is connected to the manipulating head grip. The articular section can be flexed in an arbitrary direction by way of a flexion control means (20) which is provided on the manipulating head grip. A flexible tube straightening means is provided internally of the flexible tube section to straighten the same into a rectilinear form. The flexible tube straightening means is activated into a straightening action in interlinked relation with a flexing operation of the flexion control means.
