A bag transfer device, which comprises: a conveying member that rotates in a horizontal plane, a plurality of pairs of grippers that are disposed on the conveying member and moved along an annular movement path_as the conveying member rotates, and a spacing adjustment mechanism that increases or decreases a spacing within a horizontal plane of the pair of grippers while the plurality of pairs of grippers are being moved along the movement path; and in which each gripper is composed of an arm and a clamping component installed at a distal end of the arm, the clamping components of the grippers clamp side portions of a bag, and the bag is transferred along an annular transfer path; wherein the spacing adjustment mechanism comprises: an arm support mechanism disposed at each arm of the grippers, and a drive force transmission mechanism that operates the arm support mechanism; first and second support components (8,9) are set at a base portion of each arm and separated by a specific distance in a horizontal direction characterized in that the arm support mechanism comprises: a first link that is linked at one end thereof to the conveying member and is pivotable within a horizontal plane, a second link that is linked at one end thereof to the first support component and is linked at a middle portion thereof to another end of the first link, a first restricting mechanism that restricts a movement direction of the other end of the second link such that the movement direction is parallel to the reference plane, and a second restricting mechanism that is linked to the second support component and restricts a movement direction of the second support component such that the movement direction is substantially perpendicular to the reference plane; the first link, the second link, and the first restricting mechanism constitute a Scott-Russell mechanism; and the drive force transmission mechanism transmits drive force to the second links so as to operate the Scott-Russell mechanism. a conveying member that rotates in a horizontal plane, a plurality of pairs of grippers that are disposed on the conveying member and moved along an annular movement path_as the conveying member rotates, and a spacing adjustment mechanism that increases or decreases a spacing within a horizontal plane of the pair of grippers while the plurality of pairs of grippers are being moved along the movement path; and in which each gripper is composed of an arm and a clamping component installed at a distal end of the arm, the clamping components of the grippers clamp side portions of a bag, and the bag is transferred along an annular transfer path; an arm support mechanism disposed at each arm of the grippers, and a drive force transmission mechanism that operates the arm support mechanism; first and second support components (8,9) are set at a base portion of each arm and separated by a specific distance in a horizontal direction a first link that is linked at one end thereof to the conveying member and is pivotable within a horizontal plane, a second link that is linked at one end thereof to the first support component and is linked at a middle portion thereof to another end of the first link, a first restricting mechanism that restricts a movement direction of the other end of the second link such that the movement direction is parallel to the reference plane, and a second restricting mechanism that is linked to the second support component and restricts a movement direction of the second support component such that the movement direction is substantially perpendicular to the reference plane; the first link, the second link, and the first restricting mechanism constitute a Scott-Russell mechanism; and the drive force transmission mechanism transmits drive force to the second links so as to operate the Scott-Russell mechanism.