A distal end portion of the insertion portion of the endoscope is inserted into the inside through an opening 51k on the proximal end side in the longitudinal axis direction N along the longitudinal axis direction N, and the distal end in the longitudinal axis direction N A part of the surface 51s is provided on the main body 51 formed so as to protrude forward along the longitudinal axis direction and the outer peripheral surface 51g of the main body 51, and protrudes along the longitudinal axis N of the front end surface 51s. And a protrusion 52 formed in a rib shape extending from the protruding end surface 51stm of the portion 51st toward the base end side in the longitudinal axis direction N.長手軸方向Ｎに沿って筒状に形成され、長手軸方向Ｎの基端側の開口５１ｋを介して内部に内視鏡の挿入部の先端部が挿入されるとともに、長手軸方向Ｎの先端面５１ｓの一部が前記長手軸方向に沿って前方に突出するよう形成された本体部５１と、本体部５１の外周面５１ｇに設けられ、先端面５１ｓの長手軸方向Ｎに沿って突出した部分５１ｓｔの突出端面５１ｓｔｍから長手軸方向Ｎの基端側に向かって延びるリブ状に形成された突部５２と、を具備する。
