A medical instrument for an endoscope according to the invention is the medical instrument to be used in the endoscope by being inserted therein, and the medical instrument includes: a flexible elongated sheath having a first lumen and a second lumen; a conductive wire which is inserted into the first lumen and of which a portion of a distal side is exposed to the outside of the sheath as an instrument unit; and a balloon which is attached to the sheath and is expandable with the supply of fluid from the second lumen. The balloon is configured such that an axial dimension thereof is larger than a radial dimension upon expansion and a distal end of the balloon upon expansion is located closer to a proximal side than the treatment unit exposed from the sheath.