A surgical instrument, comprising: an insertion part which is configured to be capable of inserting into a body; a base disposed at a distal end portion of the insertion part and having a bottom part provided to be extended in a longitudinal axis direction; a first grasping surface supported by the base and provided to be extended in the longitudinal axis direction; a second grasping surface provided to face the first grasping surface and configured to grasp a tissue between the first grasping surface and the second grasping surface by approaching the first grasping surface; a plurality of staples which is disposed between the base and the first grasping surface and which is capable of ejecting from the first grasping surface to the second grasping surface; a first staple line being constituted by the plurality of staples and being arranged along the longitudinal axis; a second staple line being constituted by the plurality of staples and being arranged along the longitudinal axis and elongated parallel to the first staple line; a groove part positioned between the first staple line and the second staple line in a cross direction crossing to the longitudinal axis and extending along the longitudinal axis, the groove part consisting of a first wall surface, a second wall surface, and the bottom part, the first wall surface being provided to be extended from the first grasping surface to the bottom part, and the second wall surface facing the first wall surface at a position apart from the first wall surface in the cross direction; a traction member attached to the first grasping surface and being capable of coupling with the tissue by being penetrated by the plurality of staples; a locked part (44, 47) which is a part of the traction member provided inside the groove part and having a first locked part and a second locked part, the first locked part extending along the first wall surface from the first grasping surface to the bottom part, and the second locked part extending along the second wall surface from the first grasping surface to the bottom part at a position apart from the first locked part in the cross direction; and a blade part accommodated in the groove part so as to be capable of advancing and retracting along the longitudinal axis, wherein the first locked part and the second locked part are provided in the groove part, wherein the blade part is configured to be capable of dissecting the tissue by advancing and retracting along the longitudinal axis between the first locked part and the second locked part in the cross direction, and wherein the first wall surface and the second wall surface are provided to be apart from each other in the cross direction. an insertion part which is configured to be capable of inserting into a body; a base disposed at a distal end portion of the insertion part and having a bottom part provided to be extended in a longitudinal axis direction; a first grasping surface supported by the base and provided to be extended in the longitudinal axis direction; a second grasping surface provided to face the first grasping surface and configured to grasp a tissue between the first grasping surface and the second grasping surface by approaching the first grasping surface; a plurality of staples which is disposed between the base and the first grasping surface and which is capable of ejecting from the first grasping surface to the second grasping surface; a first staple line being constituted by the plurality of staples and being arranged along the longitudinal axis; a second staple line being constituted by the plurality of staples and being arranged along the longitudinal axis and elongated parallel to the first staple line; a groove part positioned between the first staple line and the second staple line in a cross direction crossing to the longitudinal axis and extending along the longitudinal axis, the groove part consisting of a first wall surface, a second wall surface, and the bottom part, the first wall surface being provided to be extended from the first grasping surface to the bottom part, and the second wall surface facing the first wall surface at a position apart from the first wall surface in the cross direction; a traction member attached to the first grasping surface and being capable of coupling with the tissue by being penetrated by the plurality of staples; a locked part (44, 47) which is a part of the traction member provided inside the groove part and having a first locked part and a second locked part, the first locked part extending along the first wall surface from the first grasping surface to the bottom part, and the second locked part extending along the second wall surface from the first grasping surface to the bottom part at a position apart from the first locked part in the cross direction; and a blade part accommodated in the groove part so as to be capable of advancing and retracting along the longitudinal axis, wherein the first locked part and the second locked part are provided in the groove part, wherein the blade part is configured to be capable of dissecting the tissue by advancing and retracting along the longitudinal axis between the first locked part and the second locked part in the cross direction, and wherein the first wall surface and the second wall surface are provided to be apart from each other in the cross direction.