An endoscope treatment tool comprising: a sheath having a lumen that extends along a longitudinal axis thereof; a basket portion that is configured to protrude from the lumen of the sheath and that is formed of at least one elastic wire; a support member that is inserted into the sheath and in which a distal end thereof is secured to a distal end of the basket portion; and a manipulation wire that causes the basket portion to be moved forward and backward in a longitudinal direction of the sheath, wherein the at least one elastic wire has a maximum-outer-diameter portion between a distal end of the elastic wire and a proximal end of the elastic wire, and a largest portion that, between the maximum-outer-diameter portion and the proximal end of the elastic wire, reaches a maximum size in an opposite direction from the maximum-outer-diameter portion in a side view from a direction that is orthogonal to a perpendicular line (Lv) drawn to a center axis of the basket portion from the maximum-outer-diameter portion, wherein, in a front view of the basket portion, the largest portion is positioned on an opposite side from the side of the maximum-outer-diameter portion with respect to a straight line that is orthogonal to the perpendicular line (Lv) on the center axis, and wherein, in a state in which a region of the basket portion is accommodated in the sheath or a papilla, a force is applied to a portion of the at least one elastic wire between the maximum-outer-diameter portion and the largest portion in a direction in which the straight line is crossed so that an outer diameter (P1') of the portion becomes greater than that of the maximum-outer-diameter portion, the region being between the proximal end of the elastic wire and the largest portion, characterized in that the support member has a rigidity which is greater than the rigidity of the elastic wire, and which allows the support member to be bent when the maximum-outer-diameter portion whose diameter is increased is pressed by a wall of a bile duct, and the support member is configured so that, when the maximum-outer-diameter portion whose diameter is increased is pressed by the wall of the bile duct, the center axis is tilted with respect to the longitudinal axis of the sheath by bending the support member. a sheath having a lumen that extends along a longitudinal axis thereof; a basket portion that is configured to protrude from the lumen of the sheath and that is formed of at least one elastic wire; a support member that is inserted into the sheath and in which a distal end thereof is secured to a distal end of the basket portion; and a manipulation wire that causes the basket portion to be moved forward and backward in a longitudinal direction of the sheath, wherein the at least one elastic wire has a maximum-outer-diameter portion between a distal end of the elastic wire and a proximal end of the elastic wire, and a largest portion that, between the maximum-outer-diameter portion and the proximal end of the elastic wire, reaches a maximum size in an opposite direction from the maximum-outer-diameter portion in a side view from a direction that is orthogonal to a perpendicular line (Lv) drawn to a center axis of the basket portion from the maximum-outer-diameter portion, a maximum-outer-diameter portion between a distal end of the elastic wire and a proximal end of the elastic wire, and a largest portion that, between the maximum-outer-diameter portion and the proximal end of the elastic wire, reaches a maximum size in an opposite direction from the maximum-outer-diameter portion in a side view from a direction that is orthogonal to a perpendicular line (Lv) drawn to a center axis of the basket portion from the maximum-outer-diameter portion, wherein, in a front view of the basket portion, the largest portion is positioned on an opposite side from the side of the maximum-outer-diameter portion with respect to a straight line that is orthogonal to the perpendicular line (Lv) on the center axis, and wherein, in a state in which a region of the basket portion is accommodated in the sheath or a papilla, a force is applied to a portion of the at least one elastic wire between the maximum-outer-diameter portion and the largest portion in a direction in which the straight line is crossed so that an outer diameter (P1') of the portion becomes greater than that of the maximum-outer-diameter portion, the region being between the proximal end of the elastic wire and the largest portion, characterized in that the support member has a rigidity which is greater than the rigidity of the elastic wire, and which allows the support member to be bent when the maximum-outer-diameter portion whose diameter is increased is pressed by a wall of a bile duct, and the support member is configured so that, when the maximum-outer-diameter portion whose diameter is increased is pressed by the wall of the bile duct, the center axis is tilted with respect to the longitudinal axis of the sheath by bending the support member.