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