A treatment instrument includes a rotating body and a housing. The rotating body includes a connecting portion including: a supported portion having a cylindrical outer peripheral surface, and an engaged portion that is adjacent to the supported surface. The housing includes a supporting portion that is configured to support the supported portion of the rotating body, the supporting portion being rotatable around a predetermined rotation axis; and an engaging portion that is configured to generate a frictional force larger than a frictional force between the supporting portion and the supported portion by coming into contact with the engaged portion.