An advance and retreat assist tool (100) includes a base unit (300), an attachment portion (400) which attaches the base unit (300) to a treatment instrument insertion portion (35) so that the base unit (300) is rotatable around a central axis of a treatment instrument insertion hole portion (35a), and a first tubular member (500). The advance and retreat assist tool 100 further includes a rotary portion (700) and an advance and retreat mechanism (800).
