A reinforcing bar binding machine comprising: a sleeve (11, 16) having a tip end on which a hook is pivotally mounted; a long fin which is long in an axial direction of the sleeve (11, 16) and a short fin which is short in the axial direction, the long and short fins being formed on the sleeve (11, 16) at intervals in a circumferential direction of the sleeve (11, 16); a tip end shaft fitted in an inside of the sleeve (11, 16); a spiral screw groove formed on the tip end shaft; a fitting opening that penetrates from an outside to the inside of the sleeve (11, 16); a key fitted in the fitting opening and brought in mash engagement with the screw groove; a rotation stopper provided on a binding machine body and engageable with the long and short fins (33, 34); and a bumper provided between a jutting part provided on a base portion of the tip end shaft and an end face of the sleeve (11, 16); wherein, when the long fin engages with the rotation stopper, the sleeve (11, 16) advances with respect to the tip end shaft by a rotation of the tip end shaft so that the hook grasps a wire, wherein, when the sleeve (11, 16) retreats to a standby position by a reverse rotation of the tip end shaft and the short fin disengages from the rotation stopper, the tip end shaft and the sleeve (11, 16) integrally rotate so that the long fin engages with the rotation stopper to set the hook in a predetermined orientation, wherein, when the sleeve (11, 16) retreats, by a frictional force between the spiral screw groove and the key caused by colliding the sleeve (11, 16) with the bumper, the tip end shaft and the sleeve (11, 16) integrally rotate. a sleeve (11, 16) having a tip end on which a hook is pivotally mounted; a long fin which is long in an axial direction of the sleeve (11, 16) and a short fin which is short in the axial direction, the long and short fins being formed on the sleeve (11, 16) at intervals in a circumferential direction of the sleeve (11, 16); a tip end shaft fitted in an inside of the sleeve (11, 16); a spiral screw groove formed on the tip end shaft; a fitting opening that penetrates from an outside to the inside of the sleeve (11, 16); a key fitted in the fitting opening and brought in mash engagement with the screw groove; a rotation stopper provided on a binding machine body and engageable with the long and short fins (33, 34); and a bumper provided between a jutting part provided on a base portion of the tip end shaft and an end face of the sleeve (11, 16); wherein, when the long fin engages with the rotation stopper, the sleeve (11, 16) advances with respect to the tip end shaft by a rotation of the tip end shaft so that the hook grasps a wire, wherein, when the sleeve (11, 16) retreats to a standby position by a reverse rotation of the tip end shaft and the short fin disengages from the rotation stopper, the tip end shaft and the sleeve (11, 16) integrally rotate so that the long fin engages with the rotation stopper to set the hook in a predetermined orientation, wherein, when the sleeve (11, 16) retreats, by a frictional force between the spiral screw groove and the key caused by colliding the sleeve (11, 16) with the bumper, the tip end shaft and the sleeve (11, 16) integrally rotate.