Attachment unit, endoscopic insertion section and endoscope

An endoscope includes an attachment unit detachably attached to an outer peripheral direction side of an insertion section to be rotatable about a longitudinal axis with respect to an insertion main body, and including a gear portion formed over an all-round inner peripheral portion of the attachment unit, a rotary gear meshing with the gear portion to allow the attachment unit to be attached to the insertion section, and a base portion defining a gear arrangement portion where the rotary gear is arranged on an outer peripheral portion of the insertion section. In the endoscope, a channel, extended through an inside of an operation section and an inside of the insertion section from an outer surface of the operation section and opened to an outside of the insertion section in the gear arrangement portion, is defined.