An endoscopic treatment tool comprising: a sheath; a support portion which is attached to a distal end side of the sheath; a pair of forceps members (12, 13) which is placed ahead of the sheath and is supported to be rotatable relatively around a forceps rotation axis provided on the support portion; a linear portion (32, 34) which is inserted through the sheath so that the linear portion (32, 34) is capable of advancing and retracting, the linear portion (32, 34) causing facing surfaces each provided on the distal end side rather than the forceps rotation axis in the pair of forceps members (12, 13) to approach each other as a distal end portion of the linear portion (32, 34) is separated from the forceps rotation axis, and the linear portion (32, 34) separating the pair of facing surfaces from each other as the distal end portion of the linear portion (32, 34) approaches to the forceps rotation axis, wherein a common rotation axis is provided at the distal end of the linear portion (32, 34); an engaging portion which is provided in the support portion and is positioned between the distal end portion of the sheath and the forceps rotation axis; an engaged portion which is provided in the linear portion (32, 34) and is positioned closer to a proximal end side than the distal end portion of the linear portion (32, 34), and is engaged with the engaging portion when the linear portion (32, 34) moves toward the distal end side; a moving mechanism which moves the position of the distal end portion of the linear portion (32, 34) with respect to the engaged portion to a distal end position closer to the distal end side than the engaged portion in a longitudinal direction of the sheath; a first link member having one end that is rotatably connected via the common rotation axis with the linear portion (32, 34) and having another end that is rotatably connected with one forceps member of the pair of forceps members (12, 13); and a second link member having one end that is rotatably connected via the common rotation axis with the linear portion (32, 34) and having another end that is rotatably connected with the other forceps member of the pair of forceps members (12, 13), characterized in that a length (L5, L6) between the common rotation axis and a distal surface of the engaged portion is larger in in a fully open state of the pair of forceps members (12, 13) than in a half-open state of the pair of forceps members (12, 13), and the pair of facing surfaces are separated from each other by moving the distal end portion of the linear portion (32, 34) to the distal end position by the moving mechanism in a state in which the engaged portion is engaged with the engaging portion, wherein the moving mechanism comprises a male screw portion that is rotatably engaged in a female screw portion. a sheath; a support portion which is attached to a distal end side of the sheath; a pair of forceps members (12, 13) which is placed ahead of the sheath and is supported to be rotatable relatively around a forceps rotation axis provided on the support portion; a linear portion (32, 34) which is inserted through the sheath so that the linear portion (32, 34) is capable of advancing and retracting, the linear portion (32, 34) causing facing surfaces each provided on the distal end side rather than the forceps rotation axis in the pair of forceps members (12, 13) to approach each other as a distal end portion of the linear portion (32, 34) is separated from the forceps rotation axis, and the linear portion (32, 34) separating the pair of facing surfaces from each other as the distal end portion of the linear portion (32, 34) approaches to the forceps rotation axis, wherein a common rotation axis is provided at the distal end of the linear portion (32, 34); an engaging portion which is provided in the support portion and is positioned between the distal end portion of the sheath and the forceps rotation axis; an engaged portion which is provided in the linear portion (32, 34) and is positioned closer to a proximal end side than the distal end portion of the linear portion (32, 34), and is engaged with the engaging portion when the linear portion (32, 34) moves toward the distal end side; a moving mechanism which moves the position of the distal end portion of the linear portion (32, 34) with respect to the engaged portion to a distal end position closer to the distal end side than the engaged portion in a longitudinal direction of the sheath; a first link member having one end that is rotatably connected via the common rotation axis with the linear portion (32, 34) and having another end that is rotatably connected with one forceps member of the pair of forceps members (12, 13); and a second link member having one end that is rotatably connected via the common rotation axis with the linear portion (32, 34) and having another end that is rotatably connected with the other forceps member of the pair of forceps members (12, 13), characterized in that a length (L5, L6) between the common rotation axis and a distal surface of the engaged portion is larger in in a fully open state of the pair of forceps members (12, 13) than in a half-open state of the pair of forceps members (12, 13), and the pair of facing surfaces are separated from each other by moving the distal end portion of the linear portion (32, 34) to the distal end position by the moving mechanism in a state in which the engaged portion is engaged with the engaging portion, wherein the moving mechanism comprises a male screw portion that is rotatably engaged in a female screw portion.