The mounting unit 30 includes a tube main body 31 which is mounted on the insertion portion 3 of the endoscope 2 and is arranged so as to be rotatable around the longitudinal axis 3 aa of the insertion portion 3, a tube main body 31 projecting from the outer peripheral surface of the tube main body 31, And a fin portion 32 extending helically along the longitudinal axis of the main body. The fin portion 32 has a first force amount F 1 necessary for tilting the fin portion 32 toward the distal side Yf direction and a second force amount F 2 , And the second force F2 required to tilt the fin portion 32 in the direction of the base end side Yr which is a direction opposite to the distal end side, and the fin portion 32 provided in the tube main body 31 has a first force amount F1 is larger than the second force F2.装着ユニット30は、内視鏡2の挿入部3に装着されて、挿入部3の長手軸3Aa周りに回転自在に配置されるチューブ本体31と、チューブ本体31の外周面に突設し、チューブ本体の長手軸に沿って螺旋状に延設されたフィン部32と、を具備し、フィン部32は、このフィン部32を先端側Yf方向に向けて倒すために必要な第1力量F1と、フィン部32を先端側とは反対方向である基端側Yr方向に向けて倒すために必要な第2力量F2と、が異なり、チューブ本体31に設けられたフィン部32は、第1力量F1が第2力量F2より大きい。
