A guide portion 55 is provided with a gradient in flexural rigidity such that a flexural rigidity of a proximal end side on which a relatively large moment acts is higher than a flexural rigidity of a distal end side. Thus, buckling or the like of the insertion portion 15 and the guide portion 55 is appropriately prevented without setting the flexural rigidity of the guide portion 55 to be needlessly high over the entire area thereof, which enables to realize good insertability. Such a gradient in flexural rigidity enables to reduce the insertion resistance particularly in the distal end side, to realize further improvement in insertability, and to reduce the burden applied on a subject.
