The interposition section has, the observation section (14) the attachment component containment (15) the Tip which is included the bend (5) and the tubular structure (6) and. The attachment component (15) the narrow diameter portion (15b), the bend (6) Inlaying entrance it is done in the inside of the Tip side part. The bend (6) in the Tip side part the hole (8c), the hole (8c) the narrow diameter portion which corresponds (15b) the crevice (15c), it is provided in position. The screw (17a) the screw component which was formed (17), the hole (8c) penetrating completely due to the screw conclusion, Tip side the crevice (15c) is enclosed inside. Here, the crevice (15c) the diameter inside the surface which is vertical to depth direction (D), the screw (17a) major diameter (d1) it is larger.挿入部は、観察部(14)を保持する保持部材(15)を含む先端部(5)と、管状構造の湾曲部(6)と、を備える。保持部材(15)の小径部(15b)は、湾曲部(6)の先端側部分の内面に嵌入される。湾曲部(6)の先端側部分には孔(8c)が、孔(8c)に対応する小径部(15b)の位置には凹部(15c)が、設けられている。ねじ(17a)が形成されたねじ部材(17)は、ねじ締結により孔(8c)を貫通して、先端側が凹部(15c)内に収納される。ここに、凹部(15c)の深さ方向に垂直な面内における径(φD)は、ねじ(17a)の外径(φd1)よりも大きい。
