An insertion instrument which is connected to a manipulating head grip is composed of an elongated flexible tube section, an articular section and a rigid tip end section 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 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.
