A liquid feeder mounted on a treatment instrument, includes: a main tube that is arranged along a long member of the treatment instrument in which a longitudinal axis is defined and that is configured to flow a liquid to an outer circumference of the long member; a connection section that is arranged on a proximal end side of the main tube along the longitudinal axis and that is configured to connect an interior of the main tube and an interior of a liquid feed channel from a liquid source; and a fixing section that is arranged in the connection section and that is configured to fix the connection section to a handle unit arranged at a proximal end of the long member and grasped by a user.