A treatment instrument for an endoscope is equipped with a sheath； and a balloon which is provided at a distal end of the sheath, the balloon being configured to be expandable to an unfolded inflated shape from a folded initial shape by supplying a fluid, the balloon having a first region and a second region provided at both end portions in a longitudinal direction, and an intermediate section provided between the first region and the second region. In the initial state of the balloon, an amount of residual strain of the intermediate section is larger than the amount of residual strain of the first region and the amount of residual strain of the second region. When an internal pressure of the balloon has a first internal pressure value, the first region and the second region are unfolded to be faster than the intermediate section, thereby having a greater diameter than the intermediate section. When the internal pressure of the balloon has a second internal pressure value greater than the first internal pressure value, the first region, the second region, and the intermediate section are unfolded to have the inflated shape, and the diameter of the intermediate section and the diameters of the first region and the second region are substantially the same.
