A treatment instrument for an endoscope, comprising: 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, characterized in that: in the initial shape 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 when the internal pressure of the balloon is greater than a third internal pressure value greater than the second internal pressure value, a material forming the balloon stretches and expands to be greater than the diameter at the second internal pressure value. 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, characterized in that: in the initial shape 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 when the internal pressure of the balloon is greater than a third internal pressure value greater than the second internal pressure value, a material forming the balloon stretches and expands to be greater than the diameter at the second internal pressure value. in the initial shape 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 when the internal pressure of the balloon is greater than a third internal pressure value greater than the second internal pressure value, a material forming the balloon stretches and expands to be greater than the diameter at the second internal pressure value.