An attachment unit 30 includes a tube main body 31 attached to an insertion section 3 of an endoscope 2 and disposed to be rotatable around a longitudinal axis 3Aa of the insertion section 3 and a fin section 32 protrudingly provided on an outer circumferential surface of the tube main body 31 and spirally extended along a longitudinal axis of the tube main body. In the fin section 32, a first force amount F1 necessary for bringing down the fin section 32 toward an insertion section distal end side Yf direction and a second force amount F2 necessary for bringing down the fin section 32 toward an insertion section proximal end side Yr direction, which is an opposite direction of the insertion section distal end side direction, are different. In the fin section 32 provided on the tube main body 31, the first force amount f1 is smaller than the second force amount F2.
