A medical system comprising: an endoscope including a cylindrical channel; a treatment device configured to be inserted into the channel from an insertion port of the endoscope; and a power source configured to supply high-frequency power used for treatment by the treatment device, wherein the endoscope comprises a power transmission electrode installed along an outer circumferential face of the channel and configured to generate an AC electric field according to the high-frequency power supplied from the power source, and the treatment device comprises: a cylindrical insertion portion insertable into the channel; 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 the power source in a state that the insertion portion is inserted into the channel so that the distal end portion is projected from an opening different from the insertion port; a power reception electrode installed along an outer circumferential face of the insertion portion and configured to form a capacitor by capacitively coupling to the power transmission electrode when the insertion portion is inserted into the channel and the treatment section projects from the opening; and one or a plurality of protruding portions provided at a position including the installation position of the power reception electrode, wherein the one or the plurality of protruding portions is configured to restrict 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, wherein the one or the plurality of protruding portions is configured to be in point contact with an inner circumferential face of the channel. an endoscope including a cylindrical channel; a treatment device configured to be inserted into the channel from an insertion port of the endoscope; and a power source configured to supply high-frequency power used for treatment by the treatment device, wherein the endoscope comprises a power transmission electrode installed along an outer circumferential face of the channel and configured to generate an AC electric field according to the high-frequency power supplied from the power source, and the treatment device comprises: a cylindrical insertion portion insertable into the channel; 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 the power source in a state that the insertion portion is inserted into the channel so that the distal end portion is projected from an opening different from the insertion port; a power reception electrode installed along an outer circumferential face of the insertion portion and configured to form a capacitor by capacitively coupling to the power transmission electrode when the insertion portion is inserted into the channel and the treatment section projects from the opening; and one or a plurality of protruding portions provided at a position including the installation position of the power reception electrode, wherein the one or the plurality of protruding portions is configured to restrict 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, wherein the one or the plurality of protruding portions is configured to be in point contact with an inner circumferential face of the channel. a cylindrical insertion portion insertable into the channel; 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 the power source in a state that the insertion portion is inserted into the channel so that the distal end portion is projected from an opening different from the insertion port; a power reception electrode installed along an outer circumferential face of the insertion portion and configured to form a capacitor by capacitively coupling to the power transmission electrode when the insertion portion is inserted into the channel and the treatment section projects from the opening; and one or a plurality of protruding portions provided at a position including the installation position of the power reception electrode, wherein the one or the plurality of protruding portions is configured to restrict 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, wherein the one or the plurality of protruding portions is configured to be in point contact with an inner circumferential face of the channel.