The purpose of the present invention is: to ensure the proximal end part of a grip piece does not need to protrude in the radial direction of a support part, even if the grip piece is subjected to dramatic swinging； and to reduce interference with the peripheral area. A medical treatment instrument (1) according to the present invention comprises an elongated support part (2), a pair of grip pieces (3, 4) supported at the distal end of the support part (2) so that at least one of the pair of grip pieces (3, 4) can swing around a swing axis (A) that is orthogonal to the longitudinal axis of the support part (2), and a driving mechanism (5) that causes the grip piece (4) to swing. The driving mechanism (5) comprises: a cylindrical portion (8) that is provided at the proximal end of the grip piece (3), the swing axis (A) being the center axis； a driving member (7) that is rotatably supported around an axis parallel to the swing axis (A) on the inside of the cylindrical portion (8) and that has an outer surface that is caused to make rolling contact with the inner surface of the cylindrical portion (8)； and a power transmission member (6) that transmits power applied at the proximal end of the support portion (2) and causes the driving member (7) to rotate.Le but de la présente invention est : de garantir que la partie extrémité proximale d'une pièce de préhension n'a pas besoin de faire saillie dans la direction radiale d'une partie support, même si la pièce de préhension est soumise à une oscillation spectaculaire ； et de réduire l'interférence avec la zone périphérique. Un instrument de traitement médical (1) selon la présente invention comprend une partie support allongée (2), une paire de pièces de préhension (3, 4) supportées au niveau de l'extrémité distale de la partie support (2) de telle sorte qu'au moins l'une de la paire de pièces de préhension (3, 4) peut osciller autour d'un axe d'oscillation (A) qui est orthogonal à l'axe longitudinal de la partie support
