A treatment device includes: a cylindrical insertion portion inserted into a channel of an endoscope a treatment section installed on a distal end portion of the insertion portion and configured to perform treatment according to high-frequency power supplied from a power source a power reception electrode installed along an outer circumferential face of the insertion portion and configured to form a capacitor by capacitively coupling to a power transmission electrode that generates an AC electric field according to the high-frequency power supplied from the power source on an outer circumferential face of the channel when the insertion portion is inserted into the channel until the treatment section projects from the opening and a position restriction section that restricts a position of the insertion portion in the channel in a radial direction such that an amount of eccentricity equivalent to a distance between a center axis of the channel in a longitudinal direction and a center axis of the insertion portion in a longitudinal direction at an installation position of the power reception electrode is equal to or smaller than a predetermined value.
