A treatment instrument for an endoscope used in combination with an endoscope having a raising base at a distal end section of a channel, the treatment instrument for the endoscope includes a sheath section having flexibility； a treatment section installed closer to a distal end side than the sheath section； and a manipulation unit installed at a proximal end of the sheath section and configured to manipulate the treatment section, wherein the sheath section has a sheath main body having a lumen extending along an axis of the sheath section； a sandwiched portion formed at a distal end side of the sheath main body and sandwiched between the raising base raised when the sheath main body is inserted into the channel and an inner circumferential surface of the channel； and a torque transmission member extending from a proximal end side of the sheath main body to the sandwiched portion in an axial direction of the sheath section and configured to transmit a rotational torque input at the proximal end side by a user and turn the sheath main body about the axis, and an outer diameter of the sandwiched portion is larger than a diameter of a maximum inscribed circle among inscribed circles of portions sandwiching the sandwiched portion between the raising base and the inner circumferential surface of the channel when the raising base is completely raised.
