PROBLEM TO BE SOLVED: To provide an attachment unit for securing the mobility of an insertion part at the position of a lumen curve even while attached to the insertion part by applying propulsion power to the insertion part by moving a belt member.SOLUTION: The attachment unit includes a unit side rotation part for rotating with respect to the insertion part centering on a longitudinal axis by unit driving force when an apparatus side rotation part rotates while attached to the insertion part, and an annular member for receiving movement generating force to a tip direction or a base end direction when the unit side rotation part rotates while attached to the insertion part. The attachment unit includes a motion conversion part for generating the movement generating force for moving an inside movement part of the annular member to the tip direction or the base end direction on the basis of the rotation of the unit side rotation part.
