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 (P1), 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 (P2) greater than the first internal pressure value (P1), 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 (P3) greater than the second internal pressure value (P2), a material forming the balloon stretches and expands to be greater than the diameter at the second internal pressure value (P2). 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 (P1), 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 (P2) greater than the first internal pressure value (P1), 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 (P3) greater than the second internal pressure value (P2), a material forming the balloon stretches and expands to be greater than the diameter at the second internal pressure value (P2). 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 (P1), 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 (P2) greater than the first internal pressure value (P1), 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 (P3) greater than the second internal pressure value (P2), a material forming the balloon stretches and expands to be greater than the diameter at the second internal pressure value (P2). <EOS>